Some people would argue for a distinction between the legal acceptability of same-sex marriage and its moral acceptability. Consider the following case: a man leaves his wife and three kids, and runs off with a Playboy bunny half his age. He divorces his wife, and goes down to the courthouse to get a license to marry his new girlfriend. The courthouse won't ask any questions about whether he was moral or not in starting this relationship which began as an adulterous affair. They just check to see if his divorce is final, and if it is, they issue the marriage license. But if a photographer who knew about how the relationship starter, and who believes in the Seventh Commandment (Thou shalt not commit adultery) was asked to photograph the wedding, would such a photographer be reasonable in saying "I recognize that you are getting married legally, but I can't be part of the celebration of your new union, given what I know about how you got together. Sorry, please find another photographer."
(Interestingly enough, the above case seems to be one of the stronger reasons in support of same-sex marriage, since it points out that in heterosexual cases like this one, the government doesn't hand out licenses based on what they perceive to be moral or not, either on religious grounds or on any other grounds).
If that is reasonable, then could someone who object morally to a same-sex marriage do the same thing, since they are being asked, really, to be part of the celebration of something they don't feel right about celebrating?