https://usafacts.org/articles/state-relationships-marriages-...

Marriage rate declines started in the 1980s, maybe even before.

Seems to line up with women gaining financial independence, and no-fault divorce.

Related, divorce rates have declined since the 1970s:

https://ifstudies.org/blog/divorce-in-decline-about-40-of-to...

Seems like there were a lot of partnerships of necessity before, and once that necessity goes away, so does the incentive to partner up.