probably considerably less than half of the population, who are genuinely and naturally well-suited to monogamous closed relationships

If you're using "monogamous closed" here to mean "no cheating behaviour" then many studies widely report this is well under half of the coupled (presumably Western, first world) population. I'm not aware of any studies on genuinely monogamous inclination.

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 must be missing something here; I read this as a circular argument.

This is why I've always felt vaguely guilty about not being bisexual, since immortal superbeings clearly would be.

If you have more words here I would read with great interest. Again, I must be missing something, because it seems to me a similar argument could be made for immortal superbeings also enjoying every food and music type because that would similarly maximise the likelihood of obtaining food- and music-derived utility.