Julian linked to a very interesting Slate piece that asked whether opposition to gay marriage stemmed from anti-gay bigotry or a "desire to protect [the] traditional sex roles" that marriage has historically enforced. Richard Thompson Ford says that even though there are certainly a lot of plain old anti-gay bigots out there, the latter is a powerful force: "[Marriage] is one of the few social institutions left that rigorously and unapologetically divides the sexes into distinctive, almost ancient, gender roles"and that seems to be why people like it.
Anyway, Ford isn't defending traditional sex roles, he's just pointing to it as an explanation. Over at Crooked Timber, though, John Sides sifts through survey data and finds that there's something to thispeople who prefer traditional gender roles are very likely to be opposed to gay marriagebut that anti-gay bigotry is a much bigger factor.
Now there's a third part to this. Chris of the always-fascinating Mixing Memory blog wrote a few days ago on research showing that people who believed that homosexuality is a choice, rather than something immutable in human nature, were much more likely to have anti-gay attitudes. (Chris notes that there could be an upside here: If scientists increasingly discover that there's a genetic basis for homosexuality, this could create a more accepting atmosphere for gay peoplealthough obviously it could also lead to a push among social conservatives for someone to develop a "cure," ala the recent X-Men movie.)
What's also interesting here is that the people who want to preserve "traditional sex roles" through, say, marriage, are likely to think that gender is something immutable, rather than something socially constructed. In other words, they believe the exact opposite of what those who have negative feelings about gays believe about homosexuality. But it's often the exact same person who believes both things (gender essentialism and negative feelings toward homosexuality tend to go hand in hand). Basically, a lot of people are very confused.