"Rape is a crime of violence," says Gilligan. "It's a way of exerting dominance over another person and humiliating them totally. And nothing stimulates violence as much as feeling humiliated.
Aftershock of Inmate Rape - ABC News
And there is a lot of evidence/statements in like this which shows that sexual preference does not cause prison rapes, other issues often determine prison rapes. Now there are also those who choose gay sex but from what I read that usually happens to people who are locked up for longer periods of time.