No one else has a right to determine what does or does not offend YOU. Homosexuals have a right to ask if you will bake them a wedding cake. You have the same right to tell them you cannot do that. You haven’t refused them service, you only informed them you will not provide a particular service. This is not legal rocket science, is it?
He was fine selling them one of his cakes, he just didn’t want to put a message on it.