I have. Most of the arguments are either gratuitous assertion or baseless sophistry, and as you know, any gratuitous assertion may be equally as gratuitously denied.

Some of the ones I've seen:

"Marriage is for procreation"--ridiculous on the face of it, for obvious reasons. Neither requires the other.

"Homosexuality is immoral"--also ridiculous on the face of it. And even if you buy into that argument, the moral sphere and the legal sphere don't necessarily interact.

"Heterosexual marriage is a tradition"--okay, that's true. But so what? This is an example of baseless sophistry, in my opinion.

The Santorum/slipper-slope argument--fear-mongering at its worst, equating homosexuals with pedophiles. Doesn't even deserve discussion, but needs good refutation when brought up.

"We're not going to grant special rights"--argh! Protecting the rights of citizens is not granting anybody a "special" right. The current situation is the exact inverse: it's a "special" denial of rights that everyone else has.

It's worth noting that these are probably a lot of the same arguments that were made against interracial marriage, and as both product and perpetrator of same, I find them especially heinous.

I too have not encountered a valid argument, based upon true premises, that concludes gay marriage should not exist. Honestly though, I've not sought them. So now you pose the task of finding them.

Outside of my imagination, I could go to the usual suspects, those vocal protestors, strangely unified against gay marriage. (Strangely unified, because their claims thus far observed are an incoherent collection that simply testifies to the fact that the irrational and non-rational means to arrive at a conclusion are myriad. If they should converge on one conclusion, they can masquerade as one sound argument, but this is accidental at best. One could not claim they converge on one conclusion by all following the same valid process. In other words, if I were to pick any two of these protesters at random, I'll bet there's a low likelihood one could reconstruct the "argument" of the other. The "arguments" are idiosyncratically comprehended, and thus not arguments at all. This assumes their reasoning is sufficiently mutually intelligible; one need only common sense, not a priesthood, to interpret any one "argument".)

I could also seek arguments from other sources, places that may surprise me. Antithetical ones, like members of the gay civil rights community, come to mind. But let's look for sources less identifiable by logic. (I'll pop a crisp $20 bill in the mail to anyone who can construct an admissible Google query for this.)

Or maybe another question. Instead of, "What are the arguments?", how about, "What are the fatal contradictions?". One fertile area to explore is how profoundly anti-family American culture has become, and if this evolution is tied in any causal way to the rising religion of capitalism. In the long run, maybe one shouldn't expect a culture to evolve that values marriage (of any kind) and maximal profit seeking. America occupies an unprecedented position of power in the world. Like any good chemical reaction, could the energy that drives this engine derive from the broken bonds between its social atoms?