“The marriage decline happened first, though.”
It may be elites at the top of the society rejecting the Christian religion and adopting secular objectivism as the moral code began the process. Once Christian morals and values were rejected, the “glue” that held society together began to erode. Without a standard of behavior generally accepted by society and enforced by both law and peer pressure, people began shirking personal accountability. Weakening the bonds of marriage - the acceptance of “no fault divorce”, followed by the widespread acceptance of sex before marriage, and then the widespread acceptance of childbirth out of wedlock, have destroyed marriage as an institution that helps to hold society together. Men no longer have a moral or legal obligation to women with whom they procreate — the government now fulfills the role. Children conceived and “nurtured” as wards of the state become slaves of the state and will not become productive members of the middle class as they will not be taught middle class values.
Take a look at the movies from that period. They were openly promoting out-of-wedlock pregnancies, the “horrors” of illicit abortions, quickie divorces, all that stuff, long before any of it was legal.