Yes, the simplest version of marriage is a social contract, but even at that level, there are commitments made, enforced by families, which leads to warring families in the case of breach.

Hardly a model one should hold up as exemplary, worthy of returning to, and therefore irrelevant.

We don't live in a tribal society, and won't unless there's some kind of major calamity, like zombies or nuclear war or if you market fundamentalists somehow take charge and break capitalism completely.