My friend Darwin observes:
When trying to make-nice to conservatives, proponents of “same sex marriage” tend to emphasize it as a way of enshrining commitment and sexual morality. However, while this tends to suggest that same sex relationships should have the same moral obligations and boundaries as traditional ones, in practice I have never known anyone who believes that same sex marriages are moral from a Christian point of view, and yet holds that sex outside of marriage (and a host of other, related sexual issues) is definitely wrong. I’m sure that a few such people do exist, but in general even the “conservative” supporters of same sex marriage tend to have adopted a significantly loosened idea of overall sexual morality: Sex is very much what you make of it. Different people have different expectations. The key thing is that everything be consensual and that people never betray the commitments they make, whatever those may be.
This might help explain why some of the more hardline conservative Christians will remain unwilling to accept same-sex relations and recognize same-sex marriages. Christian moral reasoning is typically absolutist–i.e., based on absolute principles–and its approach to sexual moral norms is no exception. Embracing homosexuality as a valid sexual expression goes hand-in-hand with letting go of these traditional sexual norms. This is the trend I’ve seen among friends and acquaintances over the years. The absolute value of consent remains for all of them, but consent falls more in the realm of freedom/autonomy than in the domain of sexuality. Their sexual morality tends not to involve absolute truth-claims about the meaning of sex, but appeals instead to prudence or temperance since sex has ethically-relevant consequences even if it doesn’t have an absolute teleology. Like Darwin, I imagine one could find socially conservative Christians who believe in the moral goodness of homosexuality while believing also in the moral evil of sexual activity outside of a committed relationship, and they might even have persuasive arguments to buttress their position, but so far as I know, I know of none.
In this sense, same-sex marriage, as a legal and cultural enshrinement of homosexuality, does pose a threat, not to the individual marriages of heterosexual couples, but to the hold on absolute principles of traditional sexual morality. Accepting homosexuality and same-sex marriage means, in practice, rejecting most any sort of definitive sexual norms. Given how closely united these norms are to the whole Christian worldview (see for example St. Paul’s comparison of the church’s relation to Jesus with the wife’s relation to her husband), it’s no wonder that many Christians are apprehensive about embracing gay rights, even if, for personal or philosophical reasons, they wish they could do so. From what I can tell, Christians who favor gay rights also take a decidedly cafeteria approach to the Christian tradition generally, but that’s a road not all Christians are willing to travel.