Not only do girl gamers exist, but I've met quite a few in person. I know of a few from each genre of game I've played who were more than capable of
kicking my ass in that game. There is something in girls that give them an
innate ability to kick the collective asses of most guy gamers. It's pretty disheartening, to lose a one on one
deathmatch, then have the victor come over, give you a hug, and say in a sweet voice "
good game!".
It's quite possible that it goes back to the schoolboy/girl mentality of "you lost to a girl?", which is something that most schoolboys would like to avoid hearing if at all possible. There are plenty of young female athletes who are more than capable of kicking the asses of their male counterparts, and when this occurs, the boys will usually get teased (almost exclusively by other boys).
Video games, are then, the non-athlete's competitive arena. The playing field is more than leveled, as girls seem to possess a distinct natural ability for hand-eye coordination, and twitch reflexes; even while on average inferior to males in terms of strength, video games are not measures of strength, they are measures of quick thinking and reflexes.
So next time you do lose to a girl in Quake 3, Unreal Tournament, or Starcraft, don't get upset. Realize that it's more than likely that you are the one with the disadvantage. Take that "good game!" and hug with pride. And failing all else, just think to yourself:
"I let her win."