So Phishfan, if I understand you correctly:
I walk into a bakery and order a round 3-tier chocolate wedding cake with vanilla frosting and purple trim, no writing. As I'm paying for it, I mention that my fiance Jonathan will be the one to pick it up. The baker then tells me that he doesn't approve of same-sex marriage, cancels my order, and returns my money.
Do you believe this outcome is morally acceptable?
No. You ask him to mount those plastic couples or put a specific message that he disagrees with and he should be allowed to refuse.