It's not? I mean, there's some people, though probably considerably less than half of the population, who are genuinely and naturally well-suited to monogamous closed relationships. But the point that immortal superbeings would do something polyish actually does strike me as a clear argument in favor of "poly is More Highly Evolved". I mean, you're then that much closer to doing things the way immortal superbeings would do it. This is why I've always felt vaguely guilty about not being bisexual, since immortal superbeings clearly would be.