Since I'm one of those who admitted that he won't let male players play female PC's, maybe I should better explain my situation.
For starters, I'm spoiled for players in my games. The local community college has an active gaming club, so I have access to dozens of potential players. I can afford to be picky, and I am.
Since I'm gaming to have fun, I try to only game with people that are going to be fun to play with. I have no desire to play with someone who can't be bothered to bathe, or has such glaring social problems that they can't behave like a normal human in public.
So essentially, I only game with people that I know fairly well. As such, I am well aware of what kind of player they are, and try to build a game that suits them. My players all tend towards the power-gamer end of the spectrum, so I make sure to take that into account when I plan my games.
That said, I try to allow them to play what they want. If they want to try a certain class, o0r race, that's fine most of the time. However there are certain players that I have to be more careful of for the good of the group.
For example, I have one player who simply cannot handle being in a position of authority. He has a mild, meek personality. He's a great gamer, but I'd be doing the group a disservice if I let him take on the role of a swashbuckling military commander. He simply could not consistently pull it off. Likewise, one of my other players simply cannot resist the lure of power. While some players might be able to handle the responsibility of having a powerful PC, or maybe posessing an artifact, I would never give this guy any real power, because he always goes off the deep end with it!
It's the same for my "no cross-gender roleplaying" rule. I know these people. I am 100% certain that any attempt at crossing the gender lines would be disruptive, and detrimental to the group's enjoyment of the game.
Hope that clears up my stance a little.