I think it's compounded as well by the fact that a lot of "geeks" struggle with being sociable, especially with women. In a lot of cases it really is not sexism; they just don't "get" people full stop and sometimes don't even really think about the concept that someone at the other end of the keyboard is a person too. As a result they come across as highly arrogant or unfeeling (and when talking to or about women as sexist).
I've noticed on the various forums I hang out a lot of the younger types have that ingrained mild sexism that young boys usually have - only they cling to it for just a little longer. It soon goes when they head off to college :D
EDIT( you know personally I find a large number of feminists as bad or worse than sexist men - they all need throwing into a bag and shaking about a bit :P)
Lots of the 'men' in open source really are just boys with digital toys.
Sure, plenty of us have girlfriends, wives and children. But a large number do not, on /. one of the prevailing memes is that ./ers do not have a life outside of their computing.
The attitudes towards women in general, especially potential girlfriends is quite shocking.
Even on HN the stuff that comes by every now and then is beyond the pale, and even more scary, it gets voted up, suggesting the majority feels that way.