Probably the most common argument is that homosexuality is 'unnatural', and thus immoral. Of course, the same could be said of heart transplants and automobiles, so this seems a pretty bad argument to make (as I've pointed out previously; see also No Right Turn).
As for appeals to tradition, if they weren't fallacious enough already, Carl Zimmer informs us that there's some scientific evidence that our ancestors were moderately polygynous, which makes things "a little sticky for the 'one-man-one-woman-is-traditional-and-natural' camp", as he puts it. (And then of course there's all the polygamy in the Old Testament!)
Some people say they're not opposed to gay marriage as such, but think it needs to be approved democratically, rather than imposed through the courts. (Anal-"Philosopher" made some comments to this effect, if you follow the link in my previous post.) I could understand this if it were merely practical advice to gay-rights advocates, suggesting they'd have more success if they pursued other methods. But I don't see how anyone could use this to justify actively opposing gay marriage in the meantime. If others are unjustly having their freedom restricted, then this is wrong no matter what the mob thinks. Liberty trumps democracy. Besides, as legal scholar Steve Sanders points out, so-called "judicial activism" to ensure the constitution is upheld is precisely how the separation of powers is supposed to work. The judges are supposed to protect the civil rights of minorities from being trampled on by 'majority rule'. That's what they're there for.
Another common argument is that "children should be raised by a mum and a dad". There are several problems with this. The first is that there's no evidence to suggest being raised by same-sex parents actually harms children (see NewScientist). Secondly, even if it did slightly, that would not justify imposing a greater harm on adults. Thirdly, even if that were justified, prohibiting gay marriage has little effect on gay parenting, as pointed out at Alas, a Blog:
All over the country, and (outside of Massachusetts) without legal marriage, same-sex couples are raising children. They are not waiting for legal marriage, nor will banning legal marriage give the government a new right to take children away from same-sex couples. The policy marriage-equality opponents propose - banning same-sex marriage - does not in any way solve the problem they claim to be responding to, which is children growing up in homes without two biological parents.
They go on to point out that to overcome this gap in their argument, some object that "we can't allow marriage equality because that sends a message that moms and dads are unnecessary":
One problem is that it's factually wrong; equal, neutral treatment sends no message. [The full argument for this is given earlier in the post.] The second problem is that the anti-equality logic treats the lives and rights of lesbians, gays and their children as tools to be used to benefit heterosexuals.
The lives of same-sex families aren't post-it notes! If marriage equality opponents wish to send a message, they should by all means write opinion pieces, or agitate for more healthy heterosexual families on TV. But don't use lesbian and gay lives for your op-ed statement. Same-sex families are human beings, as precious as any heterosexual family; injustice to them cannot be justified by saying injustice sends a valuable message.
So far as I can tell, that only leaves the generic "gays will destroy the institution of marriage and all of society too!" objection. Such doomsday predictions have an unconvincing history, of course - similar fears were raised about women voting, women in the workforce, gays in the military, etc, but society adapted and is the better for it. What good does it do to 'protect' marriage from people who want to get married? Shouldn't those with genuine family values (as opposed to simple bigotry) be pleased that these couples are wanting to settle down in a life-long monogamous relationship?
Perhaps the best response I've seen to this objection is that of Amanda Doerty:
You have to have pretty much no belief in individual freedom to make these kinds of arguments against same-sex marriage. If same-sex marriage did have any actual connection to increases in crime, poverty, and the like, and that connection was enough reason for same-sex marriage to be outlawed, then the same reason could serve to outlaw just about anything. Divorce? That's obvious. In fact, it's almost absurd to think that someone would speak out against same-sex marriage on these grounds instead of divorce. Actually supporting divorce would be flat out hypocritical for someone making these arguments.
Besides, I would expect that legalising gay marriage could only be good for society in the long term, as with all the other progress that has been made in the past century to extend civil rights and create a more tolerant and liberal society. If you really doubt the utility of a more liberal society, go have a read of Ed Brayton's comparison of red vs blue states. It appears that those who preach about 'family values' aren't so good at living by them:
The lowest rate of divorce in the nation? That would be none other than that haven of liberal political correctness, and beacon of gay marriage to the world, Massachusetts. Must just be an anomoly, right? Well, not exactly. In fact, 9 of the 10 lowest divorce rates are in blue states, especially in the Northeast, allegedly the hotbed of pagan immorality. And the 10 highest divorce rates in the nation, with averages nearly 3 times higher than the 10 lowest? 8 of them are red states. And let's not forget that these are led primarily by what is referred to as the Bible Belt. As a recent National Center for Policy Analysis noted, "Nearly half of all marriages break up, but the divorce rates in these southern states are roughly 50 percent above the national average." Boy, I'm sure glad we've got these people to lecture us on family values, but one wonders how they get the time in between breaking up their own families!
So... are there any other (rational) arguments I've missed?
If there's anyone reading this who opposes gay marriage, I ask you: how do you justify your position? (And how do you sleep at night?)
Update: I added some more detail to the 'children' argument. (See also my new post on marriage and childrearing.) Several other arguments (including 'slippery slopes', 'opposing all state-sanctioned marriage', and the ever-present 'but I find it icky!' objection) are addressed in the comments section. The silly "marriage is defined as man + woman, so gay marriage is a contradiction" objection was discussed in my previous post.
Is there anything else?