So presumably the couple was looking to purchase a cake from the options the baker had available. There is no argument they were looking for something that was not in his portfolio, so no artistic element here. They were simply looking to purchase something he likely made for many other people.
So the question is simply, can a business or person refuse to serve another person because they believe that person will use the item in a manner that is counter to their religious beliefs? That is such a ridiculously difficult thing to parse. If the Court answers yes to that question, you would end up with all kinds of crazy scenarios. Probably even instances where the seller claims the buyer was going to use the item for a purpose that violated his/her religious beliefs and the buyer denies it.
And I would appreciate if any discussions about this case clarify that the baker is a "self-proclaimed Christian." No real Christian actually believes their religious values are compromised by serving someone else--even if they disagree with that person's beliefs or practices.