FOR THE RECORD: This answer is based on what the perception of the issue and proposed solutions are or would be. Which is why the topic is controversial in the first place.
I make no claim as to the validity of the argument for or against equal pay, this answer is simply intended to explain the reason why the concept of regulating equal pay is contentious. These are widely held beliefs and reasoning.
The position against Equal pay:
Yes, equal pay regulation is opposed, but not because anyone necessarily thinks that women should be paid less just because they have boobs, or any other reason related strictly to their gender. Basically what the feminist left is calling for amounts to wage controls. Regardless of how the people perform in their role, the proposed regulations would require that everyone at a company with the same position be paid the same rate. And if the role was similarly titled, but the job functions were different, then the regulations would require that the positions be paid the same.
So let's look at an extreme example. A company, like Waste Management(but not actually WM) has Sanitation Engineer positions.
- One of these positions is a desk job and their responsibility is to make sure that the waste collected is properly disposed of and the proper forms are filled out, and that there is a solution in place should the usual landfill/incinerator not be available.
- Another Sanitation Engineer is responsible for the collection, handling, and disposal of hazardous waste. This job requires frequently putting the employee in a position where if they make a mistake there could be serious physical consequences. In addition most of their work will involve being in an uncomfortable and restrictive Hazmat suit, and dealing with unpleasant substances.
- A third Sanitation Engineer is basically a truck driver, the collection vehicle they operate is largely automated, and has a lower paid assistant that will handle most of the physical labor, and more potentially disgusting duties.
The roles with the company mean that in theory any of the three could be call upon to perform the duties of any of the other 3 (or other hundreds of SE positions with the company). But the reality is that it's unlikely that any of these 3 would be called upon to perform the duties of the others because there are more qualified people in more similar positions that would be tapped first.
Lets assume that SE1 is female and is paid 40k a year. SE2 is male and is paid 90k a year. SE3 is female and is paid 25k a year.
The regulation would call for all of these roles to be paid the same amount. So even though neither of the other 2 are required or ever do put themselves at risk like SE2 they get the same pay rate. In addition SE1 requires less physical labor but the skills involved in the logistics are special and Neither SE2 nor SE3 are likely to be able to perform that role.
So of course this means that due to the regulations the company now needs to differentiate these roles. Meaning it now no longer has any flexibility in using SE2 in a different role should something happen and they be temporarily restricted from handling hazardous waste. It also would restrict the truck driver role from being cross trained for a move to a higher paying position. All in the name of making it fair for the sexes.
In addition it removes the ability of the company and its candidates from being able to negotiate. Maybe one candidate is willing to take a lower salary in exchange for a better benefit package, or flex time, etc. And maybe one person is more highly skilled and in demand, but because they are male should they be hired at a higher rate then the regulations would effectively give everyone a raise. It cuts a great tool for a company to excel and bring in the best people for the job for the best pay.
It really is not that most people who oppose it think that someone should be paid less because of their gender, just that imposing legislation to force equality of reward regardless of contribution is a bad idea. It is often impossible to quantify contribution and even when it is done successfully often the numbers look skewed so that men contribute more than women. This is mostly because men are in general greater risk takers are more willing to sacrifice their family time for work, while women are less likely to do both, in general. So yes often an average woman is quantifiably less of a contributor. This plays into the perception that there is a conspiracy to keep women lower paid. The reality is that women are, in general, not willing to do those things to advance their career that, in general, men are willing to do. So does that mean that as a society, we should prevent men from going that extra mile in order to be rewarded?
The position For Equal Pay:
The question specifically asked about Hillary Clintons reasoning. The exact reasoning beyond because her focus groups say thats the position she should take, is not explained anywhere I know of. I would assume its the standard reasoning I lay out below.
Four quite some time in American history, women have not held the same rights and were not treated as equals to men. Activists have been fighting to achieve equality since the 1800's.(I am sure earlier as well but refer to the organized movement)
While many of the movements goals were achieved in the 1900's there still exists, at the very least according to the hard data, some very unequal treatment in general where often the female gender is trailing behind in equal treatment. The most noticeable place is in the area of Equal Pay, also known as the Gender Pay Gap.
The position is generally that men are institutionally entrenched in management and use that position to either actively, passively, or subconsciously to exploit the psychological and physical differences in gender negotiation, performance, and needs to keep the pay of female employees much lower than their male counterparts. And the only way that this will be changed is through legislation and regulations that would ensure that women are paid equally.
"women don't deserve equal pay unless they do as good a job as men"
"You're gonna make the same if you do as good a job."
The problem seen with these arguments is that the expectations can be and often are set to advantage men when doing reviews. This means that women are rarely going to be seen as doing an equal or better job as men even if their outputs and productivity are equal or even slightly bias in favor of women.
The only way to even the playing field is seen as removing the ability to discriminate based upon evaluated job performance. The concept of Equal pay for and equal job, not performance is also a platform advanced by Organized labor. Unions have shown studies in total productivity when performance incentives are removed and pay for the position is equalized.
Note: Google "total productivity increased when individual performance incentives removed" to find your own reference there are many on both sides I refuse to champion any of them which is what a reference here is perceived to do
The belief is that a similar results would occur should pay be regulated to enforce an equality of pay among the genders. So yes the modern feminist advocates, like Hillary Clinton, oppose those statements.