I think it's great for YC to improve and help women but I hope YC doesn't start accepting female funded companies just for the sake of having more female founders instead on looking at the quality of the startups.
The only thing YC really needs to care about is 1) getting the best possible startups to apply and then 2) for those companies to be successful.
#2 is pretty much immune to pressure-on-YC; YC has no control of YC startups, with only 2-10% common equity. #1 is explicitly the thing they are working on, so it seems unlikely "we will pressure you to accept more female founders if you don't accept more female founders by convincing female founders not to apply" would be a realistic threat.
But here's another way to look at it: Accepting more women applicants -- even if they don't fit YC's acceptance heuristic (which is, by definition, already an approximation for actual success) as well -- is not necessarily a bad idea. Sexism and other irrational biases are strongly fueled by bad intuitions, and people learn these bad intuitions by not seeing enough diversity in tech and/or founders. This is a situation that YC can improve.
This wouldn't be about lowering the bar. It's more like accounting for the "home field advantage" that males have long benefited from, and eventually eliminating the advantage by letting everyone play on a neutral field starting with their first game. (First person to come up with a better metaphor gets an upvote...)
(edit: and yes, I know @sama says YCombinator won't do that. But it's still worth considering!)