Bob Kerr’s dogged obliviousness notwithstanding, the concern of marriage traditionalists is that changing the definition of marriage will have cultural consequences stretching out into the future. Of course, ever since Massachusetts’s Goodridge decision forcing just such a redefinition in that state, same-sex marriage advocates have made a point of addressing mainly strawmen and portions of opponents’ arguments that don’t risk pulling them off message.
From deep in New England, the strategy appears to be working, although other regions may allow other conclusions. Wherever one resides, however, some research explored by Charles Murray deserves a look. A study following women born between 1957 and 1964 found that, among white participants, the overall illegitimacy ratio was 11%. Dividing the group roughly 10-40-40-10 by socioeconomic class, that rate breaks down as follows:

• Overclass (17 years of education and family incomes over $100,000): 1.7%
• Middle class (family incomes over $60,000): 4.0%
• Working class (family incomes less than $60,000): 10.2%
• Underclass (fewer than 12 years of education and family incomes under $20,000: 44.5%

Murray is in the process of completing updated research, but he describes his current estimates:

Today, the illegitimacy ratio for non-Latino whites is 28 percent. How do the classes break down now? As it happens, I’ve spent the last few weeks exploring that question. I’m not done, and want to save that discussion for a formal presentation in any case, but here are some tentative estimates: The illegitimacy ratio for the white underclass is probably now in the region of 70 percent. I think that the proportion for the white working class may be above 40 percent. The white middle class is approaching 20 percent—a scarily high figure when you think about all the ways that the middle class has been the spine of the nation.
The white overclass? They’re still living in the 1950s—their ratio is probably about 4 or 5 percent tops.

I don’t know whether the “current” group includes all women (and therefore those in the previous study), but that ambiguity means that these numbers are a minimum for illegitimacy.
The relevance to same-sex marriage is that such an innovation hinders our society’s ability to leverage the institution to arrest this downward slide by erasing the link between marriage and childbirth. Whatever definition of marriage rising generations absorb from our culture, the law will tell them that it has nothing to do with the spouses’ ability to create children. Moreover, those toward the lower end of the socioeconomic ladder tend to be more susceptible to the broader culture than their better-off peers.
The women in the older study typically had their children in the ’80s and early-to-mid ’90s. That means that Murray’s new figures trace women one or two generations subsequent. Where the numbers will be in another fifteen to twenty years we can only guess. But when some long-memoried blogger like me points out declining marriage rates, increasing out-of-wedlock births, compounding teenage pregnancies, and so on, we can predict that the short-memoried public will snort at the suggestion that redefining marriage could have had anything to do with the deterioration of marriage culture.