This is where illogical folks like you fail the logic test.  He was never asked to "participate" in a same sex wedding.  He was asked to bake a cake.  Period.  He's not in the wedding.  He's not a part of the marriage. He was asked to make a dessert for the dinner party.  That's all.  It shouldn't matter whether it was for a same sex wedding or a retirement party.  Should a bakery be able to refuse to bake a cake for a bar mitzvah because their religion says Jews are evil?  Should a bakery be able to discriminate against an inter-racial couple because their religion says marriage between races is an abomination?  The answer in America is no.  You don't have to like the people you serve, but it you open your doors to the public, you can't discriminate against those in the public whose personal beliefs don't line up exactly with yours.