I secretly suspect that anyone who claims to be a gurl on the interwebz is actually a guy trying to get their jollies off of cybering, or confused young boys getting similar jollies.
Discuss.
You're not saying anything new, though.
> a lot of guys in chat rooms and forums claim to be girls
I doubt it's as common as popular culture assumes it is.
It wouldn't surprise me if a lot of the "no gurls on teh intarwubs!!1" stuff is actually being posted by girls. They're the ones most likely to notice it, after all, especially if they've been mocked in a similar manner themselves.
Consider the flipside: http://4-ch.net/dqn/archive/kareha.pl/1146511166/