You have hit on my plan. Mr G and I had this very conversation last night. Yesterday I did a wedding cake for a friend of my son. The minister was from the Universal Life (pretend) Church. Our discussion was how is this any different than serving a gay wedding?
Both are against my Christian beliefs, but I had no way of knowing that this wedding would be conducted by a pretend pastor. I would have no way of knowing if a wedding was to be conducted by a Wiccan.... or any other non Christian ceremony. What about Christians I don't agree with? If I am to stand on only serving people who don't offend my religion, should there be a religious test? Of course not.
Since I am a prayer warrior, my solution is to pray over every cake I do that whoever even takes a bite will come into a right relationship with Jesus. My cake may be the only open door given to God to allow Him to work in someone's life. Prayer works.
If my client is obviously sticking their finger in my eye (i.e. gay) then I might tell them I am praying over the cake and for all. If they believe in God, it may change their minds, if not.... oh well.... my God lives!
Think about Titus 3:10 for a moment.