I am replying against my better judgement because I really don't want to give comfort to homophobes, but I think you are making a mistake. It's not based on the prospective customer. It's based on the prospective use. Weddings are as varied as there are people who have them.
Consider a bakery which will bake cakes in the following situations: 1) A gay person purchases a wedding cake for their hetero friends. 2) A wedding cake for a het wedding that is attended by no-one but the couple and their gay friends. 3) A wedding cake for a gay couple doing a photo shoot but not being married. 4) A birthday cake for a member of a gay couple. 5) An anniversary cake for a gay couple. 6) A wedding between a gay man and a gay woman to take advantage of the tax benefit of being married.
But not the following situations: 1) A het person buys a cake for their gay friend's wedding. 2) A wedding cake for a gay wedding which is attended by only the gay couple and all their het friends.
It seems a bit weird to claim that this is based upon the protected class status of their customers. They're really only opposed to the use of their cake in a particular kind of event.