In the comments on my post about the origins of marriage, Amanda asked what impact I think the origins of marriage have, or should have, on the contemporary institution. I'd say "little or nothing."

I don't believe that institutions, or anything else, have a purpose inherent in them. There are just functions that they do or can serve at present. This means we're well within our rights to find new uses for things. I'd take as a model the process of evolution. Time and time again throughout the history of life, a feature that had been serving one purpose (say, a leg being used for walking) was appropriated for another purpose (made into a wing for flying). Humans, of course, have an even greater capacity than the natural evolutionary process for coming up with novel ways to use things that came into being for quite different reasons.

Unfortunately, the gay marriage debate is full of claims based on the "true" purpose of something (e.g. marriage, or sex). Such claims seem to me to be an attempt to give the claimant's idea of what purpose an institution should serve the stamp of a higher level of reality, a level of ideal forms without all the ambiguities of the real world. Framing "true purpose" in terms of "original purpose" serves to link the claim back into the real world and put an empirical coating on what would otherwise look like pure metaphysics.

I think the debate needs to be framed in this way: "History has bequeathed to us this institution of marriage. What functions can and ought it serve for us from here on out?" History can supply important information about what works and what doesn't, and offer suggestions as to the consequences of different arrangements, but it can't answer the value-question of which of the possible avenues we ought to pursue.