Jonathan Rauch, reviewing Red Families v. Blue Families: Legal Polarization and the Creation of Culture, by law professors Naomi Cahn and June Carbone, in the National Journal, recaps a well known fact: Republican leaning states have weak families by a variety of objective measures, while Democratic leaning states have strong ones. The analysis too is familiar.
Cahn and Carbone argue essentially that deferring childbearing to get educations has made Blue State families more economically viable and as a result more stable, and that sex without children is the only practically viable way to defer childbearing.
I'm not entirely convinced. It could be that Red State young adults are disproportionately not deferring childbearing to get educations, because they are less like to get those educations and benefit from them economically. In other words, they might just be responding rationally to their pre-existing economic prospects. Perhaps education is worth less, on average, or is less attainable in blue collar economies (often found in Red States), where fewer adults have college educations in the first place.
There are certainly many places (e.g. Italy) where very low birthrates have managed to co-exist with strong official religious doctrines that are nomimally shared by most of the population are opposed to contraception and abortion. The theory also doesn't go far to explain the greatly declining rates of teen pregnancy, even as unmarried mothers become more common.
I'm also inclined to think that there are at least three different cultural components of the Red States model: (1) an African-American community "rare marriage" model, (2) a Mormon community "marry early and multiply" model, and (3) the white Evangelical Christian community model that the discussion by Rauch focuses upon. The dynamics of the white Evangelical Christian community model may also be different in rural areas where working on a farm is a common future, and more urban ones. The dynamics and causes of each are different and are present in different proportions in different states, along with the Blue State "wait to marry and have kids" model.
The African-American community model is driven heavily by poor economic and educational prospects for young black men and economic and educational prospects for young women that are comparatively better. The criminal justice system's comparatively harsh treatment of young black men helps drive this reality.
The Mormon model has very strong religious drivers and comes with a belief (not entirely misplaced) that divorce will not be likely and an institutional safety net in the church to provide economic support to families who stay in the faith. It isn't entirely clear that this model is leading to the economic harms seen in other communities, or that it is producing weaker families. Utah has a very low average age at first marriage but very close to the median divorce rate. (Idaho, however, which has a similar age at first marriage has a high divorce rate.)
In rural areas with farm economies (which tend to be white Evangelical Christian), there may not always be much profit in deferring childbearing to get an education. Credentials matter little to young adults with careers as farmers ahead of them, who can earn an adult income quite young. Kansas has a very low average age at first marriage but a very close to the median divorce rate.
The urban white Evangelical Christian model may fit Rauch's discussion reasonably well, as may the Blue state model. But, both those models may understate the importance of key factors like the rising economic opportunities of blue collar women that is little diminished by time spent out of the workforce having children (while blue collar men have seen their economic prospects stagnate), the high economic costs of having children for well educated women (while educated people who stay in the work force receive big gains), the rise of assortive marriage (by education), and the imperfection of divorce as a remedy for economically dependent wives. In places where educated women don't face such great economic consequences for having children (like the welfare states of Northern Europe, for example) marriage has collapsed as well, although with far fewer negative consequences.
If the family as a unit is weak for several different reasons, it may take several different solutions to solve. And, the reason that marriage is actually working in some situations, may be uncomfortable ones in an age of putative gender equality.