You act as if all wedding cakes are the same, and they are not. Each one is an individually commissioned piece, the result of a contract between "willing" participants to the terms of the contract. Jack Phillips didn't refuse to sell the homosexual couple a cake sitting in his display case. He refused to be hired to create a unique item for them. Contract law is well established, it requires that all parties be voluntary participants to the terms and conditions of the contract. There is not a "universal" contract for wedding cakes. Each one is unique to the tastes and size of the party desiring to hire a baker to bake a cake for them. I would hazard a guess that there are potentially thousands of variables involved; how many layers, are the layers all the same flavor or varying. What size is the lowest layer, middle layers and top layer? 
Do you really want to live in a country where you can be forced to labor for the benefit of another against your will? I sure don't.