Because I am capable of separating the character I am controlling from myself.
I was born male, I identify as male, and I am straight. I feel no different if the avatar I am controlling is male, female, or N/A. I do not want to "bang" the avatar I am controlling even if it is a female.
Honestly I find the entire question "Why do men play a female character" to be bizarre and borderline nonsensical.
