"Masterpiece Cakeshop refused even to discuss a cake with them because Mr. Mullins and Mr. Craig are a gay couple. The bakery's owner, Jack Phillips, told them that he would not bake a cake of any kind for their wedding."
If this is the case then I would agree it's discriminatory and not protected expression to refuse service. I don't think bakers should be forced to decorate cakes in a specific theme they disagree with, though, whether it's gay, straight, Christian, Muslim, whatever. That would violate the First Amendment, and would be the government regulating the content of art.