Question title says it all really. Polygamy is illegal in most US states, here in Britain and also no doubt many countries elsewhere, but why? What are the political motivators for this, especially in a country which has a separation of church and state?
The main current purpose those laws serve is to placate a vast majority of the citizenry who object to polygamy (to the point of, about 100+ years ago, trying to physically destroy Mormons).
Ironically, this is as fully bipartisan issue as they come:
- progressive feminists see polygyny (which is what most polyamorous relationships seem to be in history) as the ultimate expression of evil patriarchy (it's beyond the scope of the answer whether they are right or not). Examples: one, two, three, four, five.
- right wing social conservatives see it as contradictory to mainline Christianity and New Testament rules (conveniently ignoring the whole King David, and especially king Solomon, precedents from Old Testament :) and generally a sexually deviant thing (it's beyond the scope of the answer whether they are right or not). Examples: one, two.
- just to make this political bed-meeting weirder (and poly? :), the less-successful (or less full of themselves) people in MRA/etc... scene also object to polygyny, as it enables women to indulge in hypergamy even easier (it's beyond the scope of the answer whether they are right or not). The only time they see eye to eye with hard-core third wave feminists, I suppose. But stranger things happened. Examples: one
Let's get down to the numbers. Even in these, extremely sexually liberal times, with polygamy being "morally acceptable" # as high as it's ever been in USA, only 17% population supports it, according to Gallup 2017 poll. That's up from only 7% in 2003.
If that seems "high" - compare it to 63% supporting gay/lesbian sexual relations.
However, an argument can be (and is) made that those laws are in fact socially a good thing, in both theoretical and practical way:
In theory, in a society with permitted polygamy, there's a high chance that a large portion of less-successful (straight) males would end up with no female partnership. This is a recipe for social disorder if there ever was one - it's well known from sociological and psychological research that males in long term relationships are far less likely to engage in risky, asocial, or criminal behavior; and are more likely to be more socially productive. Or, like, "not engage in mass violence and rioting", which probably should be considered a good thing, eh?
In practice, there is tons of research that polygamy has bad outcomes, although in all honesty, I haven't vetted ANY of the following links and don't know how solid (or biased) the research is. YMMV, PATYR.
The Perils of Polygamy has some in depth discussion (note: it seems to be a socially conservative think tank, so they clearly have an axe to grind).
The pros and cons of polygamy from Psychology Today. They often have progressive bent in my limited experience (as in, publish articles how conservatism is a mental disorder, in full seriousness) so probably also have a different axe to grind.
Slate lists a study highlighting problems. Again, progressive axe grinding is likely.
The Atlantic had an article critical of polygamy from societal standpoint, largely drawing from non-poly-related study of male-surplus Asian societies in 2004 book "Bare Branches: Security Implications of Asia's Surplus Male Population" by Valerie M. Hudson and Andrea M. den Boer.
Having said that, the above reasoning and arguments are NOT the purpose of anti-poly laws, although they may be used by defenders of such laws. The purpose stated above is the only one - to placate the majority of population that for one reason or another supports such laws.
 and if that 63% people who find homosexual relations morally acceptable seems low - that's only 6% less than 69% who find sex outside marriage morally acceptable. As in, only 6% more people don't accept homosexuality than "living in sin"
 Then again, one has to wonder at how much social acceptability bias affected that poll, since a staggeringly low 9% considers extramarital affairs "morally acceptable", compared to between 20 and 70% estimates of people actually cheating.
I'm sure someone will write a better answer but here are couple points:
Culture: Some countries just don't have a culture of polygamy. There is also an issue of who gets to marry multiple times and who doesn't, because in some places you can only do so if you are of a certain gender. It just opens a whole realm of new debates.
Administrative complexity: From an administrative perspective, keeping track of marriages is easier when people only practice serial monogamy. The interweb of marriages and the legal responsibilities that follow could become quite complex very quickly.
Inheritance: This might be the most critical reason as monogamy was originally introduced to enforce economic security. Using legally-recognized marriage as marker for which child deserves inheritance and which doesn't is an important way to keep wealth within the family. This applies to ancient monarchies as much as to real-estate owners. I would assume legalizing polygamy would create an entire industry of lawyers dedicated to sort out these issues.
Currently, anti-polygamy laws serve to "protect" traditional marriage (at least, traditional in the eyes of the Christian West) the same way that laws against same-sex marriage were supposed to. Note that these laws apply only to the legal institution of marriage, not the religious institution.
Unlike prohibitions against same-sex marriage, I doubt that prohibitions against polygamous marriage will be overturned anytime soon for the following reasons:
Same-sex marriages don't introduce the legal headaches for custody, power of attorney, property rights, inheritance, etc., that polygamous marriages would. How would you apportion assets in a poly divorce when a partner leaves? When multiple partners leave? How about parental rights?
Right now, polygamy (at least in the US) is most closely associated with some groups that are, let's face it, pretty far out on the fringe socially and politically, and it's not the polygamy that makes them so fringey.
There are arguments that poly marriages (especially polygyny, multiple wives for a single husband) truly are bad arrangements in a number of objectively measurable ways, although I don't know how solid those arguments are. But they're not the silly slippery slope arguments employed against allowing same-sex marriage ("why, next thing you know, people will be marrying their pets").
There are two very major issues with the polygamy:
- For single woman - multiple men case, it will be children that are not genetically related at all to (another) father in the family. Yet they benefit from the work, achievements, inheritance from this (second) father. Not all men enjoy this and many would insist that the woman should be only for him. It may be some pre-programmed behavior there, as many animals live in groups led by dominant male that does not let other males nearby.
- For single man - multiple woman case, seems more important that this would create a fraction of young men unable to create a family and potentially ready for everything to achieve this goal. Vikings went as far as sailing to the coasts of England and stealing girls there, DNA analysis confirm. This is only great if you recruit these men into army and send on someones head. Otherwise they will arrange a bloody coup sooner or later.
- In any case, single marriage and 50:50 % gender ratio provides reasonable guarantees that everyone seeking a partner will find one. Without these constraints, it is less likely.
The state has interest for majority of people creating families and having children, because these can provide care and support for they aged parents. While this can also be provided by various pension funds, support from children is the most natural and may be still required.
I fully back user4012 answer, the majority of the motives for outlawing polygamous relationships is not about rational arguments but tradition, religion, and 'morality' that is defined as deviation from the norm being bad. IE most people can't give you a hard and clear utilitarian argument for why they think it's wrong, it just feels wrong or it's 'immoral' without a good explanation for why it's immoral.
That being said there are two fairly rational arguments that can be made, even if this is not the real motive in most cases.
It makes marriage law harder if legal so polygamous folks should just do it on their own without demanding marriage.
The contractual relationship that is marriage is already extremely complex. Trying to extend these sort of laws to handle numerous individuals in a way that does not unfairly favor a specific demographic (see below) is not all that easy. What does a divorce mean if one person out of four leaves a marriage, how are assets divided? What if that one leaving was the primary producer of wealth, how do you handle alimony who keeps the house? How is custody handled and does the degree of biological connection to the child matter? Can a kid end up bouncing between five homes in a week because the entire marriage broke up and everyone wanted equal custody?
Furthermore marriage offers tax benefits (sort of, it's actually more complicated). How do taxes work for three people filing jointly? Currently our tax law just doesn't support that. And this isn't limited to taxes. Does my health insurance cover my entire family if I'm married to five other people and we all have kids together? Won't that bankrupt my insurance company, or at minimum result in other insurance holders subsidizing my families insurance?
It's just easier legally to only support two person marriage and let the rest have a romantic and/or sexual relationship outside the strict legal definition of marriage.
All of this would be a real hassle to sort out, but that doesn't make it impossible. Sure you can make laws and smarter tax/insurance policies and sort this all out in time. You could very well argue that this is ultimately an argument from laziness, and well it is.
But the real world is not rational or perfect. To be frank arguments from laziness work. Most people don't care about or want a polygamous relationship. Some of them may be theoretically open to supporting others having them if they want; but when you point out the cost and effort of making it legally recognized well allot of them are going to question rather it's worth all that effort to make a minority happy. I'm not saying rather or not that argument is morally right, but it is a real argument that would happen.
Still this is the lesser argument, the real argument is...
It usually results in a small number of powerful males having all the power
Here we have to discuss polygamy in theory vs practice. Polygamy in theory is a thing a number of people might support, love whoever you want to love. But when we look at it in historical practice it usually doesn't work out like the theory.
In the vast majority of cases where polygamy was practiced it usually ended up with a single dominate male having a number of wives forced into some degree of subservience to the male; how severe that was varied greatly. Often polygamy was less about "these folks all love each other" and more about "this powerful man want's another women to have sex with so he's going to pressure her into being his subservient wife"
This resulted in two issues. First the majority of males had no wives, and when 'family men' were then favored as a status symbol it would further render those who couldn't achieve wives into a lower socioeconomic class that furthered economic divides. Since everyone answering seemed to focus on the plights of lower class males I won't focus on that aspect as I figure it's already been well addressed, and perhaps even overstated, in other answers.
However things were not much better for the females. Since usually a man dominated women were often forced to join a marriage less based off of actual romantic attraction and more out of necessity. Especially the younger wives tended to lack power. Their voices would be ignored and they had to just accept decisions made by the man and possible his first few more powerful wives. Even if the women was unhappy and wished to leave she usually couldn't because she would end up without financial support for her or her kids, she may even risk losing custody and contact with her kids. The net result was that she ended up dependent on staying in the marriage even if she found it extremely unpleasant. This was rarely a good situation for the girl.
Again I stress I'm not claiming this is the case in all situations. There have even been cases of female dominated polygamy, though putting a few powerful women in charge is hardly better then putting a few powerful males in charge. I am only claiming that historically this has been the most common outcome. I also want to stress I'm not claiming that's what is wanted by most arguing for polygamy currently.
See this is the problem, right now there are people out there that desire polygamous relationships because they want to share romantic (and/or sexual) relationships with more then one person. Most of them have no desire to abuse such a marriage situation and in fact would find such a male dominated relationship as I described reprehensible. While I can't claim to be an expert in that community from what little exposure I have the majority seem to have sincere desires to simply share love with multiple people.
However, once the laws support it others will use it. Any system that exists will have at least some who will try to exploit it. There is plenty of evolutionary psychology reasons to believe powerful males would love to have their own harem of women, and plenty of historical evidence that too many powerful men treated polygamous marriage as more of a harem then an actual shared loving relationship. In a world where men already have more power then women (while we are getting better I don't think were anywhere close to an equal divide on power between the sexes) it's not unreasonable to worry if the same patter we have seen historically will occur again.
In other words regardless of the sincere desire of it's proponents who want polygamous relationships making it legally supported has a non-trivial chance of resulting in the sort of abuse we've seen in the past. It may not be the current proponents of such marriages who are most at risk of abusing them, but it doesn't really matter how the abuse comes about, if abuse of such marriages did occur it would be a problem.
Now one could argue that now that feminism has gone a long way towards fighting for women's rights and we have laws and culture that are more even in regards to distribution of power between the sexes it would not be as easy for dominate males to rule relationships as it was in the historical polygamous marriages we have seen. One can counter that the power dynamics are better but women still tend to be worse off and so men dominating such a relationship is a likely situation. You could also argue it doesn't really matter rather the person dominating a polygamous marriage happens to have an 'innie' or an 'outie' when it comes to sexual organs, the real point is that these marriages tended to be dominated by one person which lead to unfair situation for the rest in the folks in the marriage. Allot of historical polygamous relationships also involved girls forced into marriage at a young age, further ensuring they would lack power in such a relationship. Thus one might argue modern day legal polygamous marriage is less likely to have issues with power dynamics and abuse if we restrict it to only being entered into by fully consenting mature adults. All these arguments depend allot on ones specific views and I'm not going to try to claim which are stronger arguments and which aren't; frankly I don't know myself.
In fact I'm not saying that either of the main bullet pointed arguments I've made are sufficient to refuse polygamous marriage being legal. If I suddenly was told I'm in charge of marriage law* and have to decide what to do I'd be demanding far more historical records and studies to help determine how likely abuse of such a system really was and be comparing that to how many people expressed a desire for such relationships and felt harmed by the lack of such laws to try to better weigh the pro and cons of such a change. Since I lack all those studies and a sufficiently through understanding of all polygamous historical records I will not even try to render a verdict or the viability of legalized polygamous marriage.
What I will say is that I think there is a sufficient potential, and historic records, of abuse that such a risk must be reasonable considered and measured before one should say that polygamous marriage must be legalized.
And again I stress in many ways these are post-hoc justification. Most people oppose polygamous marriage not based off such utilitarian arguments and more because they think it feels wrong, 'immoral', strange, or just icky to them.
* In actuality if someone really did try to put me in charge of marriage law I would laugh in their face for putting an aromantic person in charge of marriages. I'd then ask if they also put a child in charge of setting laws for nursing homes and a bunch of old white men in charge of women reproductive rights cough. I imagine my next steps would be to scramble to try to find someone, anyone, better qualified then me to take over instead. But failing that I'd fall back on research to make a decision on such laws as I said above.
Mostly to use low-status men for the needs of wider society, rather than against it. If it's socially accepted to have multiple wives, a low-status man has little incentive to be productive and not violent. If it's assumed that some guys get multiple official wives, some of them have 0 wives despite their best attempts. That makes suicidal and violent tendencies stronger. In a monogamous society, even if some guys have more sexual partners than other's, there's at least a moral ideal of "there's a wife for everybody, because everybody should have at most one wife".
To ration females to males in equal share for everyone. Prohibition of polygamy, if combined with no sex outside marriage, will ensure the poorest ugliest dumbest males will still be able to get a wife.
Why "society" bother doing that? Well, the poorest, ugliest, dumbest males can vote. Politicians want their vote.
If polygamy are allowed, guys like Donald Trump/Bradd Pitt can get 5-10k wives, and many guys will get none. That's because men, if allowed, will aim for quantity. Look at Sultans and emperors. To prevent that, the laws "ration" females in equal share for everyone.
Yes. Women are still legal to choose poor men. However, if those women can choose to share richer smarter men instead, will they? It's like employers can still legally hire Americans. But if those employers can hire cheaper immigrants instead, will they?
Men will instinctively sabotage their competitors. So marriage laws in democratic countries allow women to pick only singles.
Matt Ridley in his book Red Queen says something along, "Far from laws protecting women ... anti polygamy statues really protect men"
Here is the quote and the whole argument
In democratic countries, the majority rules.
The majority of people are not alpha males.
So they choose to ban polygamy.
In general, we get free competition, if a small number of people compete for the benefit of majority.
For examples, most businesses compete with each other. The majority of people are benefited from their competition and hence force businesses to compete.
However, competing is not a good idea for profit.
If the majority is the competitor, we usually have trade restrictions.
The trade restrictions are in favor of majority.
For example, we got many trade restrictions in labor market. That's because laborers are majority compared to employers. Laborers are significant "voting bloc" so to speak.
Majority of men are poor or middle classes. So marriage laws in many democratic countries tend to kick out the rich from mating market.
For example, within marriage, alimony tend to be proportional to wealth of a man. This discourage richer men from getting married. Using money to get girls tend to be heavily discouraged. Explicit use of using money to get girls, called prostitution, is prohibited.
Like all marriage, the states heavily influence who can marry who.
In states that allow polygamy, who would stop the law makers from saying that girls can only marry guys that vote for me or embrace my religions? That means the religious leaders, or the alpha males, pretty much become pimp of all girls. Only by obeying the whim of those alpha males, usually called religion, a man can get laid.
No wonder, middle east often have war.