I have looked online at Sweden's website because they seem to be the most socialist country. However, same-sex couples do not have the same benefits and privileges of heterosexual couples. Isn't this reductive of socialism's message?
The premise of the question is problematic. Socialism, like democracy or communism, is an ideal. Another ideal type is to have all benefits and privileges of society extended to all people. In other words, we need to work backwards from these ideal types to first understand the current quality of socialism or human privileges protected. Then we can assess socialism's or Sweden's social democracy's relationship to the LGBT community. (Obviously, 'no-place' in the world exists like Thomas More's Utopia.)
In an ideal sense, socialism and democracy should protect all people as everyone would have equal opportunities to influence politics. However, we know that not everyone is afforded the same rights, even in highly redistributive Sweden, things are not perfectly equal. For instance, businesses and politicians have more access to information, time, and opportunities to communicate their opinions than workers who use limited time to make money in order to eat (e.g. one gets paid to shape public opinion and the other does not).
Now that we see that things are not perfectly socialist or democratic, we can begin to assess why Sweden's less than ideal social democracy may not offer the same benefits and privileges to everyone. The key insight, comes from the constraints provided by culture and history. Swedish socialism came from a long history of people developing its theoretical notions and fighting for its principles. It was run by a socialist democracy since 1917, but what that looked like was very different over the years (e.g. local income tax rates in 1930 were just under 8.5% and today they are almost 32%). What it has become and is becoming, as well as what its socialism and democracy offer to people, is shaped by its past and present.
So what does socialism or democracy offer if it is just a 'silly' ideal? Some may say that the benefit that comes with socialism is its bias toward more equal financial distribution, which will provide more political opportunities for more people. In the same vein, the benefit that comes with democracy is its tendency to have more squeaky wheels, which will provide more chances that a voice will be taken into consideration. Others may argue that more rule by the people is the goal. Regardless, socialism and democracy are two features we hope will help discriminated peoples (among many other things), but there is no complete security a philosophy or practice will provide. We are all subject to the constraints of culture and history that illustrate our values, moral frameworks, and how we interpret what socialism and democracy should look like, let alone how we can achieve them (e.g. Wendy Brown describes how liberal agendas are hampered by individualistic and group oriented demands on the state. Instead of pursuing a society ruled by the people, we pursue a society that fears the government and seeks our protection from it.)
The rights of individuals are secure or they are not, in a free society everyone is equal before the law, regardless of sexuality or gender, if you don't have that then you have fallen short of liberty and you are just dealing with degrees of tyranny.
If you are talking about a country that has a socialist government then individual rights are never secure because socialism is about subverting the ambitions of individuals to achieve the ambitions of the collective, there is nothing inherently tolerant of diverse sexuality in socialism . Sexual 'immorality' will be punished just as equally as economic immorality if that is what is politically fashionable.
In regards to your specific example, I do think it is discriminatory but necessarily so. Government licensing of marriage when looked at objectively amounts to nothing more than a subsidy of one particular type of relationship. Subsidies by their very nature have to be discriminatory to work.