I don’t have a strong opinion on this precise topic, but I have a more general, but related, comment…
I’m strongly pro-legal-gay-marriage. In fact, I think it’s an absolutely slam dunk, and no argument against it that I’ve ever heard has been even remotely convincing/logical/valid.
I also think that wedding photographers should be legally required to photograph gay weddings. But that’s far from a slam dunk. If someone says “two gay guys got married, that violated my religious freedom (and in some way damaged my straight marriage)” I sneer at them. If someone says “two gays got married, and that really bothers me for religious personal reasons, and I don’t want to be party to it by being their wedding photographer”, well, I can’t dismiss that out of hand. I’d still come down against them (and of course be bothered by their bigotry) but I think they at least have a leg to stand on.
(I think the big difference between this issue and segregated lunch counters is that the position of wedding photographer strikes me as far more connected and intimate than person-who-serves-someone-lunch. The wedding photographer is obviously providing a service for pay, but one which frequently will have a strong connection to the bride/groom and bride/groom of an emotional, and possibly artistic, nature. That strikes me as at least a reasonable, if not by itself compelling, reason for treating those two situations differently.)