You don't really have the facts quite right. He said he would sell them any off-the-shelf products, but that he would not make them a custom-made cake for a gay wedding because of his religious beliefs. He did not refuse to serve them. He simply refused to enter a contract to make a special cake. 

Because a cake can be expressive in nature and artistic in nature, I think he was within his rights to say no. Making the cake meant expressing something he did not approve of. It also meant participating in a ceremony he did not wish to participate in.

I'm hoping for a "live and let live" outcome here. Gays can marry but people who don't want to participate in those weddings don't have to. That seems a more reasonable conclusion than "bake a cake or else."