I’m fairly strictly libertarian with my views of marriage, that consenting adults should be able to associate as they like and call it whatever they choose (“marriage,” “friends with benefits,” etc.). This obviously could be called a pro-gay marriage argument. In debates with people who favor traditional marriage, the issue always eventually leads to “Well, then, what about polygamy? Would you be fine with that, you heathen?!” And yes, I generally say I would — whatever consenting adults choose to do and to call it is fine, so long as it doesn’t hamper my ability to do the same.
The argument in defense of traditional marriage is usually something referring to tradition, to majority preference, or in some cases even religion. I’ve never given much credence to any of them, but I recently stumbled on a novel take on the subject that impressed me. It doesn’t ultimately change my opinion, but it’s without a doubt the most cogent defense of “one man, one woman” that I’ve yet seen.
Marriage has been for years a way to ensure an equal distribution of males to females. Attraction develops from ancient rites of selection which favored those that were stronger, faster, and more likely to survive. However, as a requirement for society to develop, we suddenly need “experts” in various fields not directly connected to survival — i.e. the person good at farming may not be “attractive”, the person who knows how to predict the weather may not be “attractive”, and so forth. We’ll call these “beta mates”, and under a non-rigorous system they would simply never mate, and therefore have much less reason to participate in society — depriving it of their expertise.
There is another factor, as well. Historically, it has been shown that in unstructured environments, a greater number of females mated than males. The deduction to be made is that females will flock to a male they consider attractive, accepting the presence of other mates in exchange for the higher attraction and potentially stronger offspring. That we don’t see this as often nowadays is precisely because of the point I’m about to make:
Structured monogamous marriage is a method of distributing males and females equally, and provides all mates (“alpha” and “beta”) with a reward for participating in society — the “alphas” benefit from the additional expertise brought by the “betas”, and the “betas” have a very high chance of successful mating. This was for quite some time enforced through arranged marriage, and I would even make the argument that arranged marriage is what made civilization possible.
Polygamy would lead, ultimately, to alpha flocking again, and greatly reduce the encouragement for beta experts to contribute meaningfully to society. I would further argue that we have begun to see the effects of this in the USA with the considerable reduction in the sanctity of marriage and a (I would postulate) corresponding drop in technological leadership worldwide.