Again, there is a significant difference between the straw-man argument that you're outlining, and what happened here.

In your straw-man argument, it's all about the way that the cake is decorated.  You state that "racist message" on the cake.  The baker would be well within their rights to deny that cake, as long as they don't make that cake for anybody.

Now, in the case of the gay wedding cake, the has nothing to do with how the cake was decorated.  The cake decoration was not brought up at all, and the refusal to make the cake was done before any decoration had been discussed.  He could have made the exact cake for a straight couple a week ago, and he still would have denied the gay couple that exact same cake.