"Entitled to go out there and earn less than 80% of what they would get if they had been born with a penis."
Maybe men and women are not the same after all. Can we say child bearing time off for example?
If a CEO can get exactly the same productivity from an all female force, but pay only eighty percent of what he pay males, then he should be hiring as many women as possible.
Why CEO's are not doing it? I guess they are idiots, or perhaps men and women are not created equal. You already mentioned a difference that is not irrelevant to the issue. I mentioned another one that is very relevant. I am sure we can come up with some more. Those differences, whether we like them or not, affect productivity, and businesses' main goal is productivity and ultimately, profit.
By the way, I do firmly believe that men have oppressed women for millennia and continue to do so, unfairly. But in a truly free market economy the only thing that matters is profit, not gender or race.