Because I want to be paid the same as equally
performant people. I want the company to
hire equally qualified people and
pay according to equal performance.
Considering we all know hiring is an imperfect measure of skill, I would expect that pay diverges over tenure conditioned on identical qualification at hire.
What I do not want is for equally performant people to be paid differently on non-job-characteristics: so if, for instance, two people are roughly in the same bucket of performance, paying the woman less/more is unacceptable. Obviously individuals will see individual variation, but if across a number of employees, n, you see x% lower/higher pay for women than for men something is strange for sufficiently large x and n.
That is, either your hiring practices for women and men are not congruent, or they are and for some reason you're seeing poorer/better performances from women/men, then you have to identify the cause. The null hypothesis is that gender does not affect ability.