Another source of the disparity:
Men negotiate about their salary, benefits etc. when they’re hired and after. Women don’t. The squeaky wheel gets the grease.
On the other hand, a woman who negotiates is frequently seen as being “aggressive” – which is still negative for women. Men are expected to be aggressive.
So yes, there’s a double standard. But with what’s happening in the economy now, I’d guess that men aren’t doing too much negotiating either.
The bottom line: check out the going currrent salary in your field, location, level of experience, and hold out for it. A woman shouldn’t just assume she has to take what she’s offered if it’s below the going (local) rate.