A comment that Mangeek left to a recent post on marriage and polygamy merits thought and response:

I’m having a really hard time seeing what’s so bad about polygamy that it needs to be prohibited. I’m guessing that even if gay marriage and polygamy were allowed, the vast majority of people will still choose the ‘standard configuration’ we’re all familiar with.

I’d rephrase his guess: The majority of people may continue to incline toward opposite-sex pairs, at least while the cultural echo of traditional marriage continues, but that doesn’t mean that they’ll continue to enter into them — particularly not with the sense of longevity and obligation that has been the key to the institution’s success in Western society.
Same-sex marriage puts the final, irrevocable tear in the notion that the biological ability of men and women to create children through their intimacy is the single most relevant factor in marriage. Parents who pair up for life resolve questions of responsibility. They firmly set their children within lines of lineage tying them to the families and societies into which they’re born. They affirm that they are joined in the children that they have jointly made and instill a sort of existential security on which healthy worldviews and habits can be built.
If that one basic requirement is removed, marriage is ultimately about the choices and well-being of adults. Don’t get me wrong: the recent plausibility of same-sex marriage is a result of the institution’s deterioration, not the cause of it. We’ve been treating marriage as a personal lifestyle choice for decades with the common practice of the serial polygamy of divorce and remarriage. Once same-sex marriage is written into the law and thereby enforced in the culture, it isn’t even arguable that the traditional view of marriage applies except as an individual option among many.
I’ve argued many times that the whole point of responsible adults’ investing in the institution of marriage is to create a culture of marriage that draws less responsible adults toward it, thus being an active force in society, rather than a mere marker of legal responsibilities and benefits. If marriage is, by contrast, about the mutual care and support of adults, then it is a real question whether a particular woman (to pick one gender) is better off slogging through life with a peer husband or signing on as wife number 2 or 22 of a billionaire.
We’re still wrapped up in the romantic sense of marriage, so from our current place in history it seems universal and unchangeable that people will marry for love, rather than security, but it that won’t last. Indeed, creating that sense was part of what made traditional marriage a powerful force for directing our culture. Without it, not only would children not be as thoroughly intertwined with diverse and dispersed cultures (as opposed to local tribes and insular nations), but the society would drift toward hierarchies defined not merely by money and political power, but by family structure.
This is the point at which I’d bring up Russ’s comment, immediately following Mangeek’s:

Ummm, cause and effect requires that something actually has been tried (exactly what the professor above said). As Drucker put it…
“There is no ‘scientific’ way to set objectives… There are rightly value judgements … one reason for this is that the decisions stand under incurable uncertainty. They are concerned with the future. And we have no ‘facts’ concerning the future.”

Russ is responding to my suggestion that radicals/progressives have taken, as their method of operation, grand experiments with human society, basing rapid changes on the limited ability of people to foresee consequences. He doesn’t really object so much as restate his willingness to ignore the objection.
Lost in the spat, though, is the fact that tradition and cultural competition is human society’s way of experimenting over time and recording the results. Looking back at history, it appears to me, at least, that society’s that fostered a strong tradition of opposite-sex, two-person marriage became more democratic, more free, and more prosperous. That’s not a record with which we should experiment except by the long slow process of cultural adjustment, not by the fiat of politically captured legislatures, and certainly not by the declarations of unelected judges.