1) First, I have yet to meet a male player who can realistically play a female character. DM's? Sure, but they normally only have to roleplay a single character for a short period of "screentime", while a player must stay in-character for the duration of the game/campaign.
2) For me, it's very jarring to have a female barbarian who mows down opponents in a blind fury, but then giggles and tee-hee's making eyes at any male NPC. Or worse yet, who is afraid of mice and stands on a chair when one appears. I wouldn't let a player of any gender, play a PC of any gender that was just a bunch of stupid stereotypes and disruptive quirks. Sadly, this is what the majority of cross-gender PC's that I encounter are.
3) The second reason I dislike/disallow cross-gender PC's is that I find a lot of the PC's being played are nothing but flimsy masks for the player's misogny. These are the guy who want to play the cold-hard-bitch PC's, or even worse, complete and total tramps.
Granted a lot of these issues come up because many of the male gamers I play with are somewhat socially inept and tend to objectify women. This seeps out when they try to then roleplay a female PC.
Would playing a realistic female character possibly help them come to terms with the opposite sex, and maybe be beneficial to them IRL?
Maybe, but that's not my job. I'm running a game. A diversion. Some light entertainment for a group of friends and acquaintances. I'm not trying to "educate the masses."
4) The final reason, which is for me, the biggest, albeit rarest reason I dislike/disallow C-GPC's is that I am not a therapist, counselor, or avid consumer of deviant pornography. This doesn't happen often, but there are gamers who just want to either explore alternative lifestyles IC as a manner of "working out their issues" IRL.
Snipped for length