Mandating equality in outcome is probably a stance on fairness that the vast majority of Western society disagrees with. It cuts to the core of what we see as fairness. It's entirely unsurprising that this gets people riled up, though most of it is as the GP said about whether the cause is discrimination or women's own choices.

Personally I think the left in general need to grapple with the fact that we're not all equal, rather than saying that idea is tabboo because it has lead to horrible places before. The fact that the left has largely focussed on suppressing this idea that many people believe (and on an individual level is self evident) rather than tackling it head on is what leads to the backlash against political correctness because people feel like the emperor has no clothes, but they can't say so.

Not to discriminate isn't to treat everyone equally, but based on merit. If I hire/fire an employee because they're black/male/straight/christian, I'm discriminating. If I hire/fire a person because of their work performance, I'm being fair.

I don't care about the motivations of the people paying women. I care about their employee utility function, in which gender shouldn't be a factor.

Because discrimination is bad. Fighting discrimination is fighting for equality of opportunity. This makes sense and we all basically agree it's the right thing to do.

Unequal outcomes are okay. Fighting unequal outcomes is fighting for equality of outcome and it's unfair and makes no goddamn sense in a society where there is free will or any variation at all between the members.