Warning: Generalisations incoming.
Based on personal experience and testimony of others, I have to say that very often (and I'm generalising furiously here), women can be difficult to work with. Men, when pushed, can (though often don't) raise their hands and admit that they've screwed up. Women (partly because of their determination due to discrimination, I think) tend to stand much more firmly by their actions (even when they're clearly wrong). They're just as good when things go well, but getting a woman (or worse, a gay man) to admit they've messed up is often nigh impossible.
I'm not saying men never do this, but they're generally a bit less likely to when we're talking about high-level businesspeople.
That's not an excuse for lower pay, though. In fact I imagine that'd only make the situation worse. There' absolutely no reason women should be paid less than men (and let's not forget there are also some pretty incompetent men out there for whom I'd say a pay defecit is far more merited....).