Well, there are two: union and procreation. The latter is often downplayed or trivialized by apologists for gay marriage, for obvious reasons, although I realize that the distinction between the inherent sterility of a homosexual union, and the apparent sterility of the union between a man and woman who are aged or have a medical condition, can be hard to appreciate.
Earlier this evening I read a critique of "conservative" marital policy that I found tellingly sad. None of the material was new to me, but it was pretty well capsulized, which you may have deduced by now is something I find motivating. It's not the best case I've found of "I'm so openminded I must be right, and evidence to the contrary is actually evidence of a conspiracy and/or the exception that proves the rule, just like all the other exceptions," but it was convenient.
The thesis was basically that the definition of marriage has been the volleyball of elitist, xenophobic, bigoted politicians throughout history, especially Christian ones as far as those of us in western culture are concerned. Things like "binding young girls from one family to some old man from another." I guess the unwashed masses only practiced the sort of informal marriage that secular progressives teach...oh, except that those relationships were usually based around love and an expectation of children (wait, those ideas are a "Judeo-Christian innovation," which makes it the oldest concept too progressive for modern minds to accept), instead of pragmatic statesmanship and perhaps the production of a single heir, like the far less frequent political marriages that are far more important because of their rarity. Is my irony showing yet?
I was amused to see the procreation criterion being disproven by the halo effect. "Conservatives," especially "good" Christian ones, used to have miscegenation laws for social control, not because Jews or blacks couldn't interbreed with whites and therefore shouldn't try; thus, homosexuals are only prohibited from marrying because someone wants to hold them down, not because it doesn't benefit society in any graceful way.
Well, I don't think it's up to you, me, or anyone in particular to decide exactly what function other peoples' marriages should serve.
I think someone's trying to make just such a decision. You know, I don't think anyone should decide what function roads or courts or skirt steaks are for, either. It should be a decision left to Other Individuals, and you should be careful in case you make a decision for yourself that Other Individuals don't like. We're so locked down by our cultural preconceptions. Maybe we can be like the beatniks and throw off our cultural preconceptions about hygeine, too. They were so free, their doctors had to virtually turn to the history books to diagnose diseases that were thought to be moot, if not actually eradicated.
This line of thought came from ISCA, if it matters to anyone. Well, the beatnik bit was my own embellishment, but I only borrowed it from somewhere else. I used to be impressed with the level of discourse, once the edges were worn off the novelty of people arguing with their intellectual equals and it was no longer just obnoxious flaming, but now most of the discussion is between caricatures of the impressive personas I remembered. Maybe I'm outgrowing it. I hope so. I'd rather burn bandwidth and wetware on something actually important, although in some ways it's a convenient microcosm whose pulse I can keep my thumb on.
I've still never seen a good reason why gay marriage should not be allowed.
Why do I have the feeling this guy never will, either?