"A leeetle more" humility, or "I'm a little leery."
In a remarkable essay (that by itself justifies the blogosphere), libertarian Jane Galt cites three American social reforms whose negative results far exceeded critics' fears and which caution against redefining marriage today:
(1) Federal income tax. When begun, a cap (10% ?) was dismissed as ridiculous because it was so outrageously high that Americans would surely revolt before paying it. Besides, a cap would invite taxes to rise to its level: No cap = a lower tax. But, as Galt points out, "a slow creep . . . eroded the American resistance to income taxation." What are you paying today?
(2) Public welfare. The widows and orphans pensions of the 1800's expanded into temporary support for unfortunate families. In the 1950's, reformers urged expanding aid to theretofore stigmatized unwed mothers, deriding any who objected that financing out-of-wedlock conception and birth would encourage its increase. "So despite the fact that the sixties brought us the biggest advance in birth control ever, illegitimacy exploded," and public welfare finances it.
(3) Divorce. Because it was very hard to divorce in the 1800's and many experienced marital unhappiness of many degrees, reformers made divorce easier. Critics objected: "If you make divorce easier, . . . you will get much more of it, and divorce is bad for society." To which reformers rejoined: "That's ridiculous! . . . People stay married because marriage is a bedrock institution of our society, not because of some law! The only people who get divorced will be people who have terrible problems! A few percentage points at most!"
"Ooops. When the law changed, the institution changed."
As with income tax and welfare, so with divorce: The first change, tiny and incremental, made the next one easier (and with less stigma); and "the magnitude of the change swamped the dire predictions of the anti-reformist wing; no one could have imagined, in their wildest dreams, a day when half of all marriages ended in divorce"; or when (as in the 1990's) out-of-wedlock births exceeded 70% among American blacks (risen from 25% in the early 1960's); or when 35% became the de facto income tax cap (and 10% became the lowest; 2006).
Galt asks how well-meaning reformers could go so badly wrong (noting that reviled critics were right beyond even their own predictions). With regard to how liberalized public welfare undermined marriage (but with application to the other reforms, including homosex marriage), "I think the core problems are two. The first is that [reformers] looked only at individuals, and took instititutions as a given. That is, they looked at all the cultural pressure to marry, and assumed that that would be a countervailing force powerful enough to overcome the new financial incentives for out-of-wedlock births. They failed to see the institution as dynamic. It wasn't a simple matter of two forces: cultural pressure to marry, financial freedom not to, arrayed against each other; those forces had a complex interplay, and when you changed one, you changed the other."
"The second is that they didn't assign any cultural reason for, or value to, the stigma on illegitimacy. They saw it as an outmoded vestige of a repressive Victorian values system, based on an unnatural fear of sexuality. But the stigma attached to unwed motherhood has quite logical, and important, foundations: having a child without a husband is bad for children, and bad for mothers, and thus bad for the rest of us. So our culture made it very costly for the mother to do. Lower the cost, and you raise the incidence. As an economist would say, incentives matter."
And, as Galt cites G. K. Chesterton's analysis, "people who don't see the use of a social institution are the last people who should be allowed to reform it" (as opposed to deforming it). (Reading the GKC excerpt alone is enough reason to move to Galt's essay.)
This reasoning leads her to plea "that people try to be a leeetle more humble about their ability to imagine the subtle results of big policy changes. The argument that gay marriage will not change the institution of marriage because you can't imagine it changing your personal reaction is pretty arrogant. It imagines, first of all, that your behavior is a guide for the behavior of everyone else in society, when in fact, as you may have noticed, all sorts of different people react to all sorts of different things in all sorts of different ways, which is why we have to have elections and stuff. And second, the unwavering belief that the only reason that marriage, always and everywhere, is a male-female institution (I exclude rare ritual behaviors), is just some sort of bizarre historical coincidence, and that you know better, needs examining. If you think you know why marriage is male-female, and why that's either outdated because of all the ways in which reproduction has lately changed, or was a bad reason to start with, then you are in a good place to advocate reform. If you think that marriage is just that way because our ancestors were all a bunch of repressed bastards with dark Freudian complexes that made them homophobic bigots, I'm a little leery of letting you muck around with it."
While this post excerpts key arguments, please read Jane's essay and update.