It is not art. A monkey can decorate a cake.
If you offer custom cakes to heterosexuals under Colorado law you have to offer that service to all.

Jack Philips has made many cakes for gay weddings, he just didn’t know.
Only one of the betrothed gay people showed up. They created the custom cake with Jack. Jack made the cake. It was bought and paid for and then it was used in a gay wedding.

Jack Philips did not know, otherwise he would have illegally refused to make the cake.
Is Jack going to go to hell now? He made a cake for a gay wedding! Jesus must be steaming mad.

If for some reason Jack wins his case and is legally allowed to discriminate, is he goi g to ask people if the cake is for a gay wedding in advance?

Will he put up a big sign that says, “No Gay Wedding Cakes”?
Nah, it would probably be easier to just judge them by appearance and refuse service to people that look gay. They might be trying to get a gay wedding cake, sneaky gays.