Most popular opposition to same sex marriage is driven by simple homophobia. I'm sure opponents would disagree, but it seems like the rapid shift in public opinion on the question is evidence that the once-prevalent attitude that gays are weird and/or gross is dissipating as straight Americans get to know more and more out LGBT Americans.
"Gays are weird and/or gross," though, is not a great legal argument, and right now the Supreme Court is deciding whether or not there is a good reason for the state to ban same-sex marriages while allowing opposite-sex marriages. So far the best that opponents of equality could come up with was "opposite marriage is just the way marriage has always been." That falls apart when you compare the modern American institution of marriage to marriages as they've been practiced throughout human history, as anyone who has read a book called "the Bible" could tell you. So the opponents moved on to their current argument -- the last, best argument for banning gay marriage -- which is, basically, "for the children."
There is some vague indication that gay couples might be bad at raising children, but again all arguments along those lines not rooted specifically in bigotry against gay people are weak, and there is not a lot of data to support the claim gay couples raise children any worse than straight ones, or single parents.
So let's try to find the best version of the "for the children" argument. Rich Lowry is the editor of the National Review, the most venerable and prominent of the conservative magazines of ideas, and from him we should expect to find the most advanced argument against having the government grant the same benefits to gay couples that it grants to straight couples who choose to wed. His column on the subject today, though, is mostly made up of complaints that the nation is moving a bit too fast on this for his taste, and that appealing to the courts rather than awaiting success at the ballot box is undemocratic and unfair. Lowry says the Defense of Marriage Act is constitutional because it has not prevented states from recognizing gay marriage, which isn't really an argument against gay marriage. He says that if gay marriage bans are determined to be prejudicial and unconstitutional, then gay marriage opponents will face civil suits. That doesn't address the question, though, of whether gay marriage bans are prejudicial and unconstitutional.
Here's his only actual expression of an argument for banning gay marriage (emphasis mine):
In this view, the promoters of Proposition 8 came up with a definition of marriage that has stood for centuries in the West and is endorsed by every major religion simply as an imaginative way to stick it to gay people. Every serious contender in the Democratic presidential primary in 2008, including Barack Obama, supported this same definition, presumably also out of the same simmering hostility to gays.
Supporters of traditional marriage believe that the institution exists as an expression of society’s interest in children’s being raised by their biological fathers and mothers. You can say that this understanding is dated, given what has become of marriage the past 40 years. You can say that it is too pinched, given evolving mores. You can’t say it is inherently hateful.
The line I bolded is basically the last remaining "logical" argument against gay marriage, when the explicitly bigoted ones are stripped away. That line is effectively the foundation of the legal argument the defenders of Proposition 8 are currently making before the Court.
JUSTICE KAGAN: Mr. Cooper, could I just understand your argument. In reading the briefs, it seems as though your principal argument is that same-sex and opposite — opposite-sex couples are not similarly situated because opposite-sex couples can procreate, same-sex couples cannot, and the State's principal interest in marriage is in regulating procreation. Is that basically correct?
MR. COOPER: I — Your Honor, that's the essential thrust of our — our position, yes.
In other words, gay marriage is banned because the purpose of marriage, and the reason the state has an interest in involving itself with marriage, is to help ensure that as many children as possible are raised by their biological parents.
Leaving aside that allowing gay marriage doesn't prevent the state from encouraging straight parents to wed, this argument is ... horrible. Because it quickly becomes an argument for banning divorce, and for state-mandated shotgun weddings in the event of unplanned pregnancies. If the state restricts marriage to straight people to encourage them to raise children together, why would the state allow them to opt out of marriage but still have children?
New York Times' squishy social conservative columnist Ross Douthat basically demolished that argument, in a column in which he predicated his opposition to gay marriage on that argument.
Or at least, it was the Western understanding. Lately, it has come to co-exist with a less idealistic, more accommodating approach, defined by no-fault divorce, frequent out-of-wedlock births, and serial monogamy.
In this landscape, gay-marriage critics who fret about a slippery slope to polygamy miss the point. Americans already have a kind of postmodern polygamy available to them. It’s just spread over the course of a lifetime, rather than concentrated in a “Big Love”-style menage.
If this newer order completely vanquishes the older marital ideal, then gay marriage will become not only acceptable but morally necessary. The lifelong commitment of a gay couple is more impressive than the serial monogamy of straights. And a culture in which weddings are optional celebrations of romantic love, only tangentially connected to procreation, has no business discriminating against the love of homosexuals.
We are long since past the point where marriage was a mandatory child-rearing contract, and not an expression of romantic love between two adults.
If the state is interested in ensuring the welfare of children, there are a lot of things it could do that would be much, much more effective than banning gay marriage. It could provide universal childcare for working people, for example. It could spend more on nutritional food in schools, hire more social workers and pay them a great deal more,
But if the argument is that we have to go back to the day when marriage was effectively something society forced couples to do in order to make sure their children had stable homes, that argument should be made more plainly. Don't just protect Traditional Marriage, Supreme Court: Ban divorce!