If a prospective buyer came in asking for a custom cake design and there was something about that person that made me uncomfortable, I’d show him some subtly-unattractive, amateurish designs that he would be almost certain to reject. And quote an equally unattractive price.
Your remedy has been suggested many times here, and I’m surprised anyone still thinks such a tactic would work. It would be simple to prove in court what you were doing, and how you were descriminating against homosexuals.