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)