i think it has to do with equality. female gamers want to be treated like male gamers, as they should. being grouped up with a full party of girls might take away from that equality.
having said that, my wife has a few female gamers on her friends list and she occasionally has full groups of girls. they're all pretty darn good, too. getting into a lobby with a bunch of hard core trash talkers, then they hear that it was a bunch of chicks in the post game lobby, half of them usually leave without saying anything lol. i'm usually not one for trash talking, but in those kinds of situations, i can't help but laugh.
if you want to add her, go for it! her GT is mostawesomechic. she was a part of XGC a while ago as well.