The gender I pick, depends on the type of game and on the game story/setting. Most games are stereotypes, so, I don't like playing certain genders in certain games. Quick and random examples for stereotypes: Nice and gentle man is potentially a cuckold victim. Nice and gentle girl is potentially a corruption victim.
A game with more freedom would be, for example: A nice and gentle girl who can choose, if she wants to be corrupted, or have a normal love live. And a big bonus, if she can even choose, whether she wants to be interested into men or women.
These are just quick and simple thoughts and examples. I think the game type and the game setting are important for the gender choice. So, asking why someone doesn't play certain genders is trivial. Because in the end, it is just a matter of taste. And probably especially stereotypical game settings caused some tastes.