The issue is whether he discriminated against gays or whether he discriminated against "offensive art" he was being asked to "create"....just as a "vegan music composer" can decline to come up with a jingle for a hamburger chain. 
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"People" might enjoy "special class protection." "Offensive works of art" might not. Gay guy goes into his bakery and buys already-made cupcakes. No problem. But when a customer asks him, as an "artist," to design, decorate, and create an "edible work of art" that celebrates "anything" the baker finds "offensive," does "offensive art" enjoy the same "class protection status" that "people" do?