Let's be honest - this happens with men, too. But those guys become gamers anyway because with games like Dance Central, Guitar Hero, Singstar and the like, they will become sufficiently enamored with the idea of owning and playing on a gaming console that they'll come over from the "casual" scene over to what we in the industry call "actual gaming".
Why are more girls "casual" gamers than men? It could be because less games are aimed explicitly at women, while games like Guitar Hero are obviously aimed at frat boys and such. That's cool, though. Hardly expect anyone to take a risk on a for-girls product when for-everyone and for-stupid-men products rake in cash.
But mostly I think it's just because girls, well... having better attention spans than boys. Boys are basically noisy, hyperactive shits who will sit glued to a videogame screen. While girls will play a videogame, but kind of look around and want to socialize at the same time or.. whatever. Just a difference in how free time is filled. Women tend to be multi-taskers, and men seem to be glued to one particular hobby or thing at a time. That's my observation, anyway.
Re: Gender wars.
Cut it the fuck out, please.
We're all one in the same. Gender is a meaningless description applied only to separate us as humans. You could chuck it all up to "people likes what they likes," and it'd all be over, but no, it always has to cross into the realms of "sexism". Both "sides" do this, and its childish and embarrassing. So please, just... god. STAWP EET