In line with the data we have for divorces, we see dramatically increased rate of out of wedlock childbearing over the same time periods being discussed:

https://www.cdc.gov/nchs/data/nvsr/nvsr48/nvs48_16.pdf

This suggests to me that couples are still coupling, just not inviting Uncle Sam to the party. Given that other users have shown a wealth divide for marriage, it would seem that poor couples are opting out of marriage in order to obtain/keep local/state/federal resources, e.g. SNAP, Medicaid, etc, which almost always have income caps for eligibility.