Before you all shout down my neck at me being sexist. It's my views get over it.
Anyways.
I think there are plenty of reasons it doesn't have women. The first being, TF2's theme is meant to be non-serious and generally quite amusing all while having fun game-play (See 'Sandvich' for details) and Statistics show that women are less funny. Don't yell at me - look on the TV, the majority of good comedians are men. I don't know why it is but people seem to find men funnier for whatever reason. The point stands that men are considered funnier and hence a better fit for the game genre.
Secondly, I think when valve created the game, they didn't want people to worry about political correctness in any way shape or form. I mean one class mocks one-eyed people, black people AND the Scottish all in one person (in good humored way of course), if they'd want to use stereotyping for a girl, they'd either have to make her extreme at one thing or another to keep with the theme, all the classes of the game would be damn hard to pull off to keep to the stereotype and be a girl, the only classes with real potential would have to lose their old personality, I.E Medic(made German doctor) > Nurse(overly-blonde) or Scout(cocky smartass) > Female scout(Tomboy with a nasty attitude).
No matter what way you look at it - tf2 does not 'need' a girl for political correctness or whatever other reason - the genre works just fine. The games characters are some of the best throughout all of valves games. Best leave them as they are.