I think I'm just going to voice my personal experiences since other people seem to have made all the important points and I don't really want to repeat things.
Anyways, I'm a female but I'm able to play both genders. I try to balance out my female/male roleplays because when I do that I find that it's easier for me to distinguish different characters and personalities than if all my characters were, let's say, female.
Even when playing a female character in a somewhat-romantic roleplay involving another male character, I personally do not find it odd if the person who plays the male character is a female. Likewise for if I were playing a male character with a female.
I think maybe it's because I use quite a bit of male characters that I just grew to distinguish the characters from myself. Even with female characters, I never, ever see myself in them.