TF2 is about soldiers repeatedly fighting over pieces of land. Not very "feminine".
I dare say, if TF2 tried to make a female character, it would have to be sexist as all hell just to fit in with the "feel" of the game.
There are other differences between the characters in TF2 other than their nationality that do not get their own sterotypes. Why is it necessary that as soon as a woman appears, sexism is necessary? Why isn't the reverse true? Do you think that it may have something to do with male being considered a "default" gender?
I'm a male gamer, and I don't mind. (in particular, because the gameplay is so damn awesome).