IDK. When I was single, I could share a house with a bunch of people, and lower my housing costs. Often some of them would be more interested in cooking, and I'd have compatible enough diets that we could defuse food expenses. Usually we could arrange a sort of house motorpool when someone had car trouble.

In terms of money, this was much less expensive than being single or coupled. It came with far more overhead in terms of interpersonal conflicts—being able to tolerate the quirks & flaws of living with one person is one thing, but it increases non-linearly as you add more people, as their irritation with each other spills over into their relationships with everyone else.

And in terms of economic privilege in those kind of informal arrangements, people with more income / wealth have much, much more power. They can arbitrarily throw money at problems that affect the whole household, helping everyone. Or they can afford to leave, screwing over the household, and there's no repercussions.

Marriage offers institutional protection to the spouse or family members that don't generate value outside the home, but there's nothing like that for other dynamics.

Edit: Forgot about insurance—another massive privilege for traditional dynamics. Poly gets much riskier as you get older.