Overall I don't know why, being a girl gamer myself I don't see anything wrong with it.
I think it's because gaming is seen as a 'guy' thing as well as it being geeky/nerdy, so other girls think we are acting tomboyish on top of being nerds and geeks. Face it we make fun of people who do nothing but study. Also we catch the same flak from some guy gamers, probably from shock and awe as we own them
On that topic: My boyfriend gets heat from a few people for playing games, so I assume guys suffer the same.
I don't deal with this issue much because my profile doesn't hint at my gender and I'm a ghost online. I don't talk much online to avoid this situation. Some are cool with it, while other will flame you by saying that I suck or whatnot. So I try to avoid people like that, who just talk crap.
My friends mainly play games as well, so I get no problems from them about it. With my friends who don't, they don't really talk about games with me or I with them about games...
Unless they ask me what I think about a game so they can get it for their boyfriends =P