i think a part of the legal analysis that's missing in these cases is that from a legal perspective, marriage has pretty much nothing to do with sex. you can get married and never have sex, you can get married and have extra-marital affairs, whatever, the state no longer seems to be generally concerned with such matters, except possibly of the couple wants to get a divorce.
given that, what's the "sincerely held religious belief" about when it comes to refusing to print up *wedding* invitations? where in the bible does it say it's a sin to get *married* to someone of the same sex? i get that there may be some (bigoted) religious objections to having *sex* with a same-sex partner, but getting *married* to someone of the same sex is a different question.
moreover, where in the bible does it say it's a sin to print invitations for someone else's marriage? you're not participating in homosexual activity when you print up a marriage invitation. again, as noted above, you can't even be certain there's any homosexual activity in the entire picture. yeah, it may be a pretty good guess in most cases, but point is, we're now several steps removed from the actual "sin".
so now they're objecting to doing business with a customer who might be committing something they think is a sin, when these people are invariably from a religion that believes that everyone is a sinner anyway, yet they'll happily do business with all the other sinners....