I don' see from where you conclude that " immortal superbeings would do something polyish" . Why it is not as likely that they will evolve to have a series of monogamous relationships? The science of falling and staying in love is even now quite well understood . All it takes is few hormones. (see http://www.youramazingbrain.org.uk/lovesex/sciencelove.htm and the references therein). By using them only when with your partner you can make a love relationship monogamous relatively easy.

That said, do you have references for " though probably considerably less than half of the population, who are genuinely and naturally well-suited to monogamous closed relationships" ? If the monogamous love is determined by hormones, which have been in humans for millions of years doesn't it make it more likely that nowadays and few millions years future humans are more likely to be monogamous. A possible explanation for the "polypartners" people could be that because of the abundance of choice they are likely to make wrong decisions (see Human motivation :(http://lesswrong.com/lw/71x/a_crash_course_in_the_neuroscience_of_human/) I would expect future humans to be busy with more interesting and challenging things then finding the next sexual partner.

By the way you can experiment to hack yourself bisexual by trying to fall in love with a man with the three simple steps described in the end of the reference.