Feminists

Unionization would provide a framework in which to address the problem. On the flip side, it would also reduce some of the incentive to really excel in one's field. Today if you're really good, you can get paid more. If everyone with the same level of experience is getting paid the same, then the financial incentive to excel is gone. There would probably have to be a lot more flexibility in management handing out titles so that the people really good at their jobs could get a 'higher' title faster and thus get paid more.

Also, be a little wary of some of those surveys. Often they consider jobs the same just by title, and not years of experience on the assumption that experience is baked-in to the title. That assumption isn't true - one of the trends in business is to reduce the number of titles and thus a wide variety of experience gets stuck into a single title. One "Senior Software Engineer" may have 6 years experience, while another has more than 20. Same title, but the latter is far more valuable and thus far better paid.

With a statistically-significant number of women taking time off for child-rearing, that cuts down on experience. Good studies will match experience as well as title to correct for this. And they still show a gap.

I don't know how much of that gap is pure sexism and how much of that is other factors (time off, less aggressive in career, etc). Addressing the issue will depend on what exactly the cause is, and I'm not sure anyone's done sufficient study to slice-and-dice the information such that root causes can be found.