The structure of a business like a bakery is quite different from that of a portrait artist. A bakery requires far more in volume to run than just what it would get from referrals. But say you did implement a referral system, then what do you do if one of your friends personally recommends a gay couple?
Every business that is open to the public is expected to abide by public policy. The business owner can stand up for his beliefs, but the point will come when it will cost him dearly.
I remember being taught as a little Sunday School girl that the day will come when we will suffer for our beliefs. I think that day is upon us.
None of my existing art patrons would recommend a same sex couple, but if they did, I would do the same thing, refuse and tell them I don't feel like it.
A bakery might require more in volume, but they get that, they sell cupcakes, cookies, pastries, and any gay or straight can walk in and get their bakery products. Just like any gay or straight could walk into my shop and buy something off the wall.