Male/Male
In
SL, I have total control of what I look like, so I can easily default to a guy av.
In a lot of other games (notably medieval/fantasy RPGs) I chose the gender of the characters I think are rendered better. In most of the cases the guys are way too beefcake for me.
Some games, like Oblivion (or Fallout 1/2), pin specific gameplay mechanics on genders, so I would go with what I'd have the most fun with, instead of what may look the best in vanity mode.