Several months ago Jane Galt wrote a very thoughtful piece on gay marriage. A libertarian, Jane looked at the standard arguments over whether or not introducing gay marriage would destabilize traditional marriage and have unforeseen and unwanted societal impacts.
Most interesting is the angle she takes in addressing the standard canard: "How would allowing gay people to get married ruin your marriage? Are you going to want to go out and fornicate or commit adultery or get divorced just because these two nice men are allowed to marry each other?"
Her answer is, of course it won't de-stabilize her marriage, nor probably yours either. But what it could very well do, is change the balance of forces acting on the marginal cases, people on the dividing line, unsure of whether to get married before having children, or unsure of whether to save sex for marriage, or what have you.
One of the examples that she uses runs as follows: Eighty years ago, society's treatment of single mothers was just terrible. Widows, sure Churches and city pensions helped them out. Women abandoned by their husbands also got a decent amount of help. But if you were unmarried and had a child, you could expect to have your life and your child's life made miserable for your sins. And certainly you couldn't expect any outside financial help unless you went to a home for wayward girls and put your child up for adoption.
Was that charitable? No, often not. And in the 50s and 60s progressives began to push to provide state benefits to impoverished unwed single mothers and their children, just like the state already did for widows and orphans. I mean, come on, what woman would set herself up for the life of hardship that is single motherhood for a poverty line state benefit?
Well, the answer, as a statistician or evolutionary biologist should have been able to tell them, is that it would be attractive to those women who were worse off than that to start with. (What good is half an eye? A bit more than one quarter of an eye...) In other words, if a woman's future is either a) likely to end in single motherhood anyway or b) likely to be at a lower income than welfare payments (or at least, not much more, and at the expense of much harder work) then the existence of welfare payments for single months will drive up out-of-wedlock births.
Sure, other things came into play as well, primarily the general social/moral meltdown of the sixties, but the existence of welfare for single mothers undoubtedly led to higher illegitimacy rates in the inner cities over the last forty years. And in that sense, perhaps the suffering and stigma inflicted on single mothers and their children 60+ years ago was actually less than the suffering that has result from removing the stigma.
There's no way to go back on this. Much as societal traditionalists might like to turn back the clock, you can't artificially produce a social stigma (except in a subset culture whose mores you control). But as we contemplate future social/moral developments, from same sex marriage to cloning as well as the continuing impact of past changes such as legal abortion and plentiful contraception, we need to put on our Darwin hats and ask: what selection factors are we creating, and what selection factors are we taking away.