Economic inequality my ass. Number of pre-marital sexual relationships is a far better indicator of divorce rates than economic inequality. And of course divorce of leads to single parenthood.
Correlation does not equal causation.
Also, it's not like the only possible social norms are either: no premarital sex, never get divorced, OR have lots of premarital sex and have tons of kids out of wedlock.
How about a social norm that says: use birth control and take responsibility if you choose to have children? That might also work.