He should have asked them to decorate the cake with something positive about traditional marriage. I doubt if the bakeries even took the request seriously. What bride would want a cake that says “gay marriage is wrong”? But she might want one that says “Marriage between a man and a woman is a divine institution” or “One man, one woman” or something similar. And he should introduce himself as very conservative, very traditional, and very religious. That would make a more equivalent situation.
That's what I was thinking. The message needs to be stated in a positive way that is anathema to gays.