I've been pondering and attempting to apply pop-psychology to this "there are no girls on teh intarwebz" category of reactions. (Disclosure: I am an adult male.)
Given the difficulties verifying another person for gender/hawtness, it's easier to say "girls don't exist here" and continue a gender-neutral discussion. Nobody has an opportunity to cackle "Surprise!" or mock anybody else for gullibility, desperation, etc. On one level, it's a triumph of feminism in that with minimal effort you will be treated the same as a guy in almost all topics of conversation.
On the flip side: "Someone who might understand my areas of interest instead of shoe shopping and teen soap operas?" is a powerful lure.
And of course we're on the continuing trend of the internet away from techies and towards popular consumption. So certain geeky segments of the population have to deal with the fact that the same mechanisms leading to more girls also lead to more superficial jocks. (Related to
this piece of rage.)