That's not sexism, though. That's women negotiating less.
That means there's likely some kind of societal pressure keeping them from negotiating as much. There have been plenty of studies that show assertive women are seen as just being bitchy while assertive men are seen as strong leaders.
That societal difference is sexism. Individuals don't have to be consciously sexist for implicit bias to affect women.
If you want to argue that it's sexist that we socialize women to negotiate less, i'm all for that, though.
If that's the road you feel comfortable walking down, I think it's fine to compromise at that point. Regardless of how it happened, the result - women not negotiating - is still the same.
Women generally don't handle negotiating the same as men. But negotiation skills aren't relevant to most jobs. So if that's a factor in how people get paid, it's _systematically_ sexist.
Because sexism or no, I think it's a tremendous problem when people get ahead for completely tangential reasons to their job performance and skills.
Unless your point is that the sexism is at a larger cultural level that fosters the average woman to be less effective at salary negotiations. In that case I completely agree.