First of all. I dont RP at all.
I dont take so much consideration in to the gender. My main during my WoW time was a female night elf druid. My melee chars tends to be male while my casters rather often is female, especially healers. I never assume peoples gender based on their characters gender. When it comes to pixels is no more different than hair color or skin pigment. It shouldnt matter what gender, skin color, hair length your character have. Just how you play it.
IF a would play on a RP server, I might put some more thoughts into my character, as that is what you suppose to do, and you should be able to relate to that char, and then it would be difficult for me to play a female character, but if someone choose to play their opposite gender, it is totally up to them.