I haven't weighed in on this yet. I was thinking about it because a commenter at another site described the conservative viewpoint that SSM will hurt traditional marriage as "just silly" and "impossible to defend coherently" without an appeal to religious values.
Let's go at that first, not because it is a powerful argument, but because it is a common one. Mere dismissiveness never impresses me with its explanatory power. It must work on a lot of people, however, because it keeps coming up. Pish tosh. Piffle. No intelligent person believes that. Get real. Sniff...sneer. It is often an announcement that the speaker has talked about, read about, and heard about the subject, but not actually thought about it.
I will touch on arguments for SSM that might persuade me, or at least be worthy of consideration, a bit farther down. But this one is frankly amazing. Every known society to date, even polygamous ones, has not sanctioned same-sex marriage. If you travel to Papua New Guinea and ask several tribes at random "Say, have you ever thought of having men marry men, or women marry women?" you would receive puzzled stares. If you traveled in time and place to the Congo River Basin in the 6th C, an Inuit community in the 12th C, or a Brythonnic tribe in the 28th C BCE, you would find no societies that allowed this. Perhaps they were all wrong. Perhaps it is a great advance in rights or attitudes to be considering such a change in Western Europe and North America in the 21st C. But to dismiss the universal practice of all human beings to date as "just silly" seems, well, silly.
There have been cultures that have allowed and even encouraged homosexuality. The Greeks are most often cited on this, but there have been a few others. None of them has same sex marriage. In polygamous societies, the additional spouses are seen to be in some contractual relationship with the opposite sex partner, but not with each other.
One might find this universal vote from history to be insufficient or unpersuasive to forbid SSM. But by what blind ethno- and chronocentrism can one discard it as silly? I trust my examples have also set to rest the complaint that forbidding SSM is an idiosyncratic Christian idea. We should be suspicious when people try to sail these things past us.
The strongest positive argument for SSM comes from an idea of individual rights. As with many other practices in the American experience, one could maintain that people have a right to do what they want to do regardless of its effect on society, or more mildly, unless society can show some compelling interest in forbidding the act in question. Voting, owning guns, using drugs, moving where one wishes, zoning laws - all these involve some discussion of whether individual rights are the trump card. It is worth mentioning in passing that progressives who argue from some inalienable right to SSM that they have some serious counterarguments to deal with, not so much from conservatives as from libertarians. Societal provision of some good, such as health care or food, is made on the basis of the more communitarian idea that "I want to live in the sort of society that..." Well, I might want to live in the sort of society that worships squirrels, or insists everyone marry at 16, or sings for its supper, but so what?
There is not only a specifically religious set of arguments against homosexuality per se (I don't know that SSM is separately addressed in the texts of any religion), but a set of traditionalist arguments that many religious people also assent to. The people holding both sets of ideas may confuse them - surely they seldom differentiate them clearly - but they are nonetheless not identical arguments. The traditionalist says "This is the way things have always been and it seems to work reasonably well. To make a change in that, the burden of proof is on the advocates for SSM that it will be okay." The counterargument from individual rights that something should be allowed unless good reason should be shown for its forbidding - the declaration that the burden of proof lies on the side of the forbidders - has merit. But any declaration that SSM might be good for any other reason automatically admits the claims of the traditionalists as well.
The libertarians who support SSM do so entirely on the basis of individual rights. If progressives go down that road, the libertarians have a host of additional rights they will want to press if we are to be consistent. If progressives choose to go down any other road to get to SSM, they will find themselves unable to answer the traditionalists with anything other than majority opinion or naked power. I don't think anyone wants to go there.
As for the evidence that SSM will damage traditional marriage, well, what sort of evidence would we accept? If SSM's had half the divorce rate (20%) of heterosexual marriages or twice the divorce rate (80%) neither would be much evidence one way or the other that traditional marriage had been affected for good or ill. If traditional marriages got worse or got better, there would be very little way to show that SSM were even a partial cause. It is necessarily indirect, and perhaps impossibly subtle to discern. I might claim that the environment for my child is very different if my neighborhood is 0%, 20%, 90% SSM, but what data could I base that on?
This is actually a strong argument for SSM from one perspective. Opponents are going to be hard-pressed to offer evidence, because what evidence would even theoretically be admitted? Perhaps with a perspective of decades, with some countries allowing and some not, we could tease out that individual factor. But that strong argument carries its own weaknesses. It is equally difficult to give evidence it is a positive, or even harmless. At most, we might show over decades that there was little change, or only changes which could be attributed to other trends.
This would at first seem to be a standoff, with libertarians claiming that such ambiguities must always be decided in favor of the individual right, while the traditionalists (and communitarians, though they won't like it) arguing that societies have a collective right to create the environment they choose. This one wants a society where everyone has free health care, this one wants a society where gay people are celebrated, this one wants a society where everyone works, this one where Christian holidays are celebrated, this one where sharia law is practiced, this one where education is really, really supported, this one where alcohol is forbidden... All fine until we come up against the question of what if somebody who lives here doesn't want that? Or doesn't want to be made to pay for it? The SSM advocates then go into the same pile as everyone else competing for societal design.
Two final points, both a bit tangential. My view is that SSM would be mildly damaging to an overall set of societal values, including traditional marriage, but not very much. If I were a legislator I would vote against it, if I were a governor I would veto it, but I wouldn't put my time into any protests or advocacy. Marriage hinges on a lot of other things related to the two participants well before it hinges on what's happening with other people's marriages. I am far more concerned about our denying cultural inheritance. Every place where we make a different decision than our own ancestors, we cut ourselves off from them a bit more. Far from being multicultural, such responses are monocultural, believing that only our own time and place have the answers. I believe that we have made some changes that are positive - that we are more right than our ancestors on some things. But I give up each of those coins only slowly, after much consideration. I am very unwilling to believe in the general wonderfulness of my own time and place versus other times and places.
Secondly, I react with great suspicion when the advocates for a position use their worst arguments almost exclusively.