Well, let me see if I can explain my viewpoint on this where it will make sense to people. First, some background, to eliminate any vagueness. I am 40 years old, and have been gaming for just over 25 years. I am not only comfortable in my own sexuality, but I am comfortable with others expressing their own sexuality. The gender or sexual preference of a player has no effect on whether they are part of my game group. Currently I have 9 players in my group (plus me). There are 3 women, 6 men (plus myself), 2 of the players are married to each other, 2 are getting married this year, 2 are an unmarried couple, 2 are married to non-gamers, and myself and 1 of the other men are single. At least 2 of the players are bisexual (one of each gender) and 1 of the women is bi-curious. None of the players are anywhere close to being homophobic or even uncomfortable around homosexuals (male or female).
All that being said, I do not allow players to start a campaign as a character of the opposite gender. If one of them loses a character and decides to create a different character, I am not adverse to letting them play an opposite gender character. The reason for this is simple. When starting a campaign, especially with a group this size, the DM needs as few confusing bits as possible. Knowing that I can associate the character gender with the player gender allows me to quickly work in certain elements of a campaign without constantly having to check things.
As an example, I had a roughed out idea for a starting encounter, one where the characters come together during the first session. One of the NPCs would only trust some information to another woman, because she was suspicious of the men of the small town. This is for good reason, as the men were being controlled by a necromancer who was using the men to create an army of zombies. So, knowing that she would not trust men I could easily roleplay the encounter with the players by talking only to the female players and being shy and evasive towards the men. There was no mental notes required that I have to include "Bob" in the group of females because he is playing a female character.
It made the encounter easier for me to adjudicate. Until I got used to running htis big of a group the same gender rule jst made things easier for me. Juggling class and race information for 9 players is bad enough, at least this way tring to remember gender as well was not an issue. Not everyone sits with a picture of their character propped in front of them*, so remembering details about a character can be difficult for spontaneous role-playing.
So, call me lazy, call me a prude, whatever floats your boat. I insist that players play their own gender purely for selfish reason.
As a side note, out of the hundreds of characters I have seen played by dozens of players, I can count on one hand (and still have fingers left) the times where a player has played an opposite gender character in a manner that was not stereotyped, excessively expressive of their gender (for no good reason), or was just an expression of the players dislike of the gender.
* Actually, I don't think I have ever seen anyone do this in all the years I have been playing.
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