> In most places, living with someone is basically the same thing as 'marriage' from a legal perspective.

If this is a reference to common law marriage, it's not actually that widespread anymore (only in 10 states in the US). It also typically requires the couple "hold themselves out as married" in the community, which makes the bar substantially higher than just living together for a long time.

If this isn't a reference to common law marriage, I'm not sure what it is—feel free to enlighten me!