The "specific message" he is against that he will make wedding cakes for everyone. He wont, obviously. He only wants to make them for straight couples. That seems to literally be his only requirement. Which, is textbook discrimination.

Again, the cake they were asking him to make was a normal wedding cake.  No writing on it.  He is suggesting that just the process of making a cake that is used in a gay wedding, goes against his beliefs.  So, if he can say that, why couldn't anyone else say they wont sell someone a product because they dont agree with the couple being interracial?  Or Jewish?  Or Muslim?  Its a slippery slippery slope. 

Making a cake that "promotes" something is a little different, in my opinion.  That would include writing or specific decorations.  For Halloween, for example, "Happy Halloween" or a picture of a Jack O Lantern.  For THOSE things, you cannot force an artist to create something they are against.  I agree with that.  

But this situation is different.