Very true—what libertarians should do is promote common‐law marriage, which is permitted in 9 states. Unfortunately, over time fewer and fewer states have allowed forming new common‐law marriages. And in those states where it’s still possible, same‐sex common‐law marriage has always been illegal. So “get the government out of the marriage business” still requires active effort to change the laws.

https://en.m.wikipedia.org/wiki/Common-law_marriage_in_the_U...