Throughout history we've had single marriages, polygamy, second+ marriages, combined families, divorce, matriarchal societies, and patriarchal societies. And there was certainly a time before any kind of marriage at all existed, and it's possible nobody knew who any fathers were. Throughout all of that, the individual is the only common denominator.

There is no reason to think that a single marriage is the best format, other than it happens to work well within the framework we've built up to support it over the last few hundred or few thousand years in specific countries. So the individual is it, because it's common to all.