My friend raised his points around this, they were rather well thought out.
A) Most people wouldn't like playing (or even seeing) female characters unless they fit a certain stereotypical body type (thin, not too tall, busty.) This would severely screw up one of the most intuitive parts of TF2: The silhouettes. It would be hard to make them easily distinguishable and still fit the aesthetic requirement most male players would demand.
B) You. Don't. Hit. Girls.
Yes I know it comes off as sexist, but it's true. Most males have an inherent inability to hit a girl. In my friend's words "I see a spy, I shoot him. I see a female spy, I feel a twinge of guilt... which might cost me my life." Alternatively, if the roles are reversed, you don't really feel guilty putting a knife into a heavy's back. He's the big dumb slow guy. If it was a girl, and you hear her scream as you stab...
Those are a couple reasons why female characters would cause issues for this game.
Personally, I have nothing against it. I'd quite like to play a female scout. Possibly designed like