Oh, for Christ's sake (literally, for the sake of Christ). If he finds others so repulsive he can't imagine servicing them, then he should withdraw himself from society so he doesn't have to interact with them. This of course speaks poorly of his faith. But aside from that, typically the couple tell the person what they would like. I seriously doubt they would have said, "just come up with something for us," as they gave each other a big wet kiss in front of him. Usually the couple tells the baker what they want it to say.
And aside from that as well, since Mr. I'm too good for you Christian Baker man doesn't want to serve people who challenge his narrow beliefs, and since the law of this land says you cannot discriminate and deny services to the public based on race, religion, or sexual orientation, then he should close up shop and go join some cloister where he doesn't have to confront his prejudices against other humans. It would be a grand opportunity for him to learn about Love. He should have the gay couple come and teach him what he as a Christian fails to apprehend from his own religion.