[cryoshon]

In practice biologically, sure, you're right, but it's tough to look beyond the tribal-cultural ingrainment of marriage into people's psyches... sure, people may always fantasize about doing better than they currently are, but in how many people will those thoughts turn into actions?

There's too many contented or at least not actively dissatisfied people to support the idea you're discussing in practice, though I agree it's theoretically plausible.

Rather than a question of sexual access relative to resources, I'd reframe it for the 21st century as a mutual understanding of shared needs and shared solutions to those needs. Sure, not everyone can fill every need for every other person, but it's probably better than going without, which is why the pragmatic mate choice also provides genuine positive emotion, even if it's not the "maximum" need fulfillment possible.