The issue is marriage between first cousins. When I first saw people ranting about it online, I was surprised, because not only did I have no objection to it, I hadn't realized that it was a cultural taboo in some areas. Half the states in the Union prohibit first cousins from marrying, though. Someone in a thread online speculated that the states which prohibit cousin marriage also would be hostile to same-sex marriage, with the implication that those stupid inbred hillbillies are hypocrites or something. Cousin marriage is illegal in several states that could be viewed as stereotypical, though: Alabama, Kentucky, and West Virginia, for example. And a number of the commenters who feel compelled to let the world know the grossness of cousins marrying are nevertheless nice liberal advocates of same-sex marriage.
Here in Indiana, first cousins are permitted to marry as long as both are at least 65 years old. In much of the world, though, cousin marriage is not only legal but preferred. The common rationale for prohibiting it is that the children of cousins are more likely to have birth defects. That's true, but the total risk is still pretty low. Besides, what evidently bothers most of the people I see ranting cousins about marrying isn't the genetic risk: they're just grossed out by it, an objective fact too obvious to be debated.
Which gives me an excuse to post here a story I've been sitting on for years. A decade ago I was using a university computer lab. The only other people there that day were two young women, both students, talking loudly to each other as if they were using cell phones. I wrote down some of their conversation as it happened and sent it to myself in e-mail, so I can vouch for the accuracy of the dialogue below:
I left in my "retarded" reaction, even though I know it's an unacceptable pejorative, because it's essential to the plot.Two girls are sitting on the other side of the lab from me discussing their love lives, and one is telling the other how she met a guy her younger brother's age (2 years younger than she is) who turned out to be a fourth cousin. "We're religious and stuff", so she considers this too close for them to get romantically involved. The other one is telling her to go for it: "Third cousins would be too close, second cousins would be too close, but I think this isn't too close. ... God musta wanted you guys to meet," said the other. "You really think so?" said the first dubiously.
Her brother teases her about their having retarded kids. Her friend says, "Well, he's your brother, of course he's gonna tease you."
"But what if we wanted to get married, I mean like how gay is that?" Oh, girl -- the only risk your kids run is that their mother is retarded.
Fourth cousins? I'm amazed that the family tree was that well-recorded. I wasn't even aware of my second cousins when I was growing up, and if I'd met some random person who was a third cousin I don't know how I'd ever have figured out the relation. And what religion considers fourth cousins, or third cousins, too close? Even first cousins aren't forbidden to marry in the Bible. (It's worth noticing that among the forbidden unions in that list are people like stepsisters, who wouldn't even be related genetically. Whatever the basis for these prohibitions, it's not a concern for birth defects.) According to Catholic tradition (i.e., fantasy), Joseph and Mary were first cousins, and there are numerous other cousin marriages in the Bible, none of which is condemned.
So, it's clear that whatever the source of this revulsion I'm seeing toward first-cousin marriage, it's not religion. That would be true even if I weren't an atheist who doesn't consider religion to be the source of anything: religion is a stew into which people toss their hangups and virtues, and get them back endowed with the yummy goodness of divine authority. Parents can't necessarily be blamed either, since then you have to ask where the parents got their taboos. By the same token, even devout religious believers are often capable of ignoring biblical or other religious prohibitions.
One good thing about the same-sex marriage controversy, then, is that it brings attitudes like these to light. But is anyone paying attention to them? I said "controversy" rather than "debate," because there's very little debate. Most Americans, including the college-educated, think that debate means stating their opinion and stopping there; but that's just where debate begins.