Why religious people are against gay marriage.

But Witherington, a conservative scholar, goes on to point out that all sorts of sins involve the implicit rejection of God or His commandments and that homosexuality is on par with covetousness, malice, envy, murder, slander, insolence, rebellion against parents, ruthlessness, deceit, pride, and the like—not one destined to destroy society. (And liberals, of course, believe the Bible does not discourage homosexuality at all.)

In other words, many of the world's faiths do argue against homosexuality, but they don't raise it to the level of moral calamity: It's bad but not that bad. Privately, religious conservatives are appalled and grossed out by homosexuality but realize that the more common American view is modulated. So, they choose to focus on the idea that marriage in general is under threat. Read their public statements, and you'll see a surprising shortage of outrage about homosexuality itself. Perhaps they've been reading their Bibles more carefully. More likely, they've figured out that the most effective argument for religious conservatives is not, in fact, a religious one.