Your question isn't about discrimination.  It's about a product that the baker doesn't offer, and does not involve discrimination.  The baker is able to decorate the cake any way he wants, and refuse to decorate a cake any way that he wants, as long as he consistantly does it.  If the homosexual baker doesn't bake the cake that says "Sodomy is Evil" for anybody, then it's not discrimination when he doesn't bake it for a conservative.  Now, if he will bake it for Muslims, but not Christians, then you get into discrimination.