Like some people, it depends:
1. Which class looks better. In TO, I've played both genders. Only in Atlantica did I choose a male char because the girls looked crap.
2. What IGN I'm using. I have several IGNs that I use on rotation, so if it's a male/female one in particular, then obviously I go with the appropriate choice o_o
Those are pretty much the only 2 factors I consider xD