Many porn agents are extremely reluctact to book their girls (especially their top talent) into "alternative" scenes – things like transsexual scenes, bisexual scenes, etc. They usually do it under the ruse of "protecting the girl’s career" or "health concerns."
The first one is complete garbage of course. A brief list of girls who have participated in one of the sorts of scenes mentioned above: Belladonna, Tory Lane, Roxy Jezel, Brianna Love, Dana Vespoli, Rebeca Linares, Jayna Oso, Delilah Strong, Kate Frost, Shy Love, Isabel Ice, Ariana Jollee. Two of them went on to sign exclusive contracts (Roxy & Tory). One of them is one of the biggest stars the business has ever seen (Bella). I would hardly say working with shemales or gay guys hurt their career.
As for health concerns, it is pretty common knowledge that plenty of "straight" male performers spend some time (on or off camera) with shemales or gay men. I could turn this into a out-fest, but that’s not the purpose of this piece. You know the names.
I love when I hear new girls says, "I would never have sex with a gay guy." Oh yeah? You do it almost every day on set.