If one is in the business of baking cakes, what possible difference does it make what the purchaser does with it? What if the buyer wants it for a food fight? What difference, so long as the baker is not asked to participate in the food fight or, in this case, attend or officiate at the wedding?
If the purpose or use of a product is to be a factor in its purchase, as in guns and drugs, the baker will need to question all prospective purchasers to determine if they are "qualified buyers."
"We reserve the right to refuse service to anyone" used to be a very common wall adornment behind the cash register of many local businesses. Like so many social mores of the past, perhaps this one, too, should be superseded by a business policy of treating everyone equally and fairly, regardless of one's opinion of their private lives or beliefs.
Mr. Baker, bake the cake, take the money and send them on their way. You have more important dough to contend with.