Ah, the debate of decades...
It's certainly true that a lot less girls seem to play games than guys, I have a few theories...
1. Girls tend to do lots more socially. Maybe this is by virtue of being a girl, but growing up I always found that girls would go shopping, walk around aimlessly talking, go to the cinema, just spend the day at each others' houses doing whatever came to mind... whereas boys tended to go for playing sports, going to the pub, or cramming five of themselves onto a two-seater sofa with a consignment of XBox controllers. If I ever played games with my non-gamer girl friends, it was usually fifteen minutes of "oh you have to see this, it's so cool", after which we'd go get a drink and sit in the garden laughing at strangers.
2. Girls often get bored of FPSs and RTSs. I can't stand FPSs for the most part - there are only so many ways you can shoot someone, and after a few minutes, you run out, and it just feels like a grind. If you're going to play a game in which you kill people, you might at least do it with style - sneaking up on someone and knifing them is much more satisfying than shooting them from behind a Conveniently Placed In The Middle Of Nowhere wall of sandbags. I love RTSs, but I know most of my female friends get instantly turned off by anything describable as a "war game".
3. Many girls are quieter in games. We don't feel the need to confirm the presence of our breasts by calling anyone else with them a dyke - so there goes a large percentage of ingame chatter

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4. I forgot this one.
5. I forgot this one too.
6. Most of the girl gamers I know adore RPGs. A few play MMORPGs, but a lot just play one HELL of a lot of Oblivion. I guess there are less multiplayer RPGs than other games? It pretty much rules out XBox Live for a start.
Well as said, all these apply to *most* girls *I* know, so you can stop typing "I'm a girl and this is all BS!!!" now - but just a few thoughts.
As for the people who suggested it was mostly social influence.. well I can't comment on now, because now, at the age of 16, I look at small children and think "Their parents should be shot. wtf is that?? This kid is five! It should be playing with xyz, not smoking on my front garden!". But when I was a kid, it was definitely the case that boys played runescape and girls played the Sims. Not sure why, but the sexism theory holds water.
(Incidentally, I was saved from this stereotype by joining a Sims 2 forum, becoming friendly with a few staff/veteran members; who then proceeded to inform me that they were staff because they loved the forum, and hated the game. Then they held my head under the water of C&C, Morrowind and Dungeon Seige 2 for a while, and I emerged choking in disgust at my own previous bad taste in games.)
Female programmers - I know about four serious (ie professional) male programmers, and two female ones. Not sure if this is generally accurate but it fits the stereotype.