If this guy was a freelance baker who did not have his own shop open to the public, I would agree with you.  But no artist that sells their art in a public way (like out of a gallery...or out of a bakery, in this case) has any say over where the purchaser of that art, places that art.  

This "art" was also nonspecific art.  It was no different looking than any other past cake he had made for any other wedding.  This was not a rainbow cake, and he was not even asked to put 2 Grooms on top of it.  It was a celebratory cake if anything - nothing really special or unique about it in terms of how it looked.