“Muslims religious conscience now trumps Christian religious conscience?”
They have argued that there is nothing in Christianity that would prohibit baking things for a gay wedding and if you use that as an excuse why dont they refuse cakes for other sins or divorced people. I think fighting it on a personal freedom argument would be more effective. Better yet, I have advocated telling the couple you would make them a cake, but would then contribute their payment to an anti-gay marriage organization. That should dampen their enthusiasm to do business with you.
Alternatively, a contract requirement that the buyers display placards by the baker — just like many construction and renovation companies require — that say something like:
Proudly Baked By Alpha & Omega Wedding Company — May God Bless the Union of This Man and Woman in Holy Matrimony
A hefty liquidated damages clause for damage to reputation for failing to display or for defacing the sign, along with notice that the profits will be sent to a pro-traditional marriage group would stop the problem.