They would have to officially state in their bylaws that people who participated in gay porn could not compete. That would open up a huge can of worms. After all, they allow former convicts and people known to use recreational drugs to compete.
And I'm pretty sure that the IFBB would have to ban participation in ALL porn, and not make any specific reference to gay porn, due to that being a form of discrimination. Even trying to ban participation in all porn in general (other than illegal forms like kiddie porn) would probably open up a floodgate of all sorts of legal and other problems for the IFBB. And if you are somehow able to take away the porn actors, ex-cons, and rec drug users / dealers, it may be pretty hard to find and keep competitors.