I'm a bit of an alpha male (I know, that'll shock some of you) but I don't normally have too much of a problem with opposite-sex players in longer campaigns. There has been one VERY notable exception, sadly, where a ... male... was playing a supposedly female character who was a vengence she-bitch who was raped and wants to kill all men, etc... The player was so unpleasant that he was kicked out of the college gaming groups.
I've had a few other players that were jerks when playing women, but they 'got over it' pretty fast when other women were around, or I simply said "You do realize I won't be giving your character any experience for this adventure due to this, right?" Problem solved after that. But, again, these were very much minority experiences.
In general though, as a GM, I prefer that players keep to their gender just to make things easier on me, particularly if I'm in a situation when dealing with variable groups that will swap characters and players a lot. But this isn't a problem per se, but more a convienience for keeping things straight.
"Thank god I'm only watching the game... controlling it!"