I play male if relationship with NPC differs with the sex (see: Persona 3 Portable, Dragon Age, Harvest Moon,...), or female if it doesn't have any influence on gameplay/story.
In the first case I'm more roleplaying as the character himself, in the second, it's more directing, I can identify to her.
In the first case I'm more roleplaying as the character himself, in the second, it's more directing, I can identify to her.