Well, it's a bit of a premature-observation-cheat. But Isabella did mention that Brian and Jackson could make arrangements with a female-female couple to get kids all around. In principle, this ends with everybody having the same number of offspring and replicated genes as in two male-female couples. Arguendo, why couldn't that lead to evolutionary equilibria that were just as stable?
Counter-arguendo (Thellim replies to herself): Assuming both Brian and Jackson cooperate to raise both of Brian and Jackson's respective children, and the two women do the same with their two children - there'd be a lot of guardian-investment in children not genetically related to the bearer. It seems like over evolutionary time, you'd get guardians showing favor to their related children over their unrelated children, and maybe slipping some help to their offspring being raised elsewhere. Which seems like it would degenerate back into heterosexual pairbonding.
For that matter, wouldn't some female-female couples go looking for unusually cheerful and healthy geniuses to inseminate them? And wouldn't that produce dramatically greater reproductive variance for males than females, which would come with its own set of problematic evolutionary trends over time? Or would that act of optimizing their children count as a forbidden-to-think-about concern over genetic inheritance because people in Isabella's world start murdering people as soon as they think about that, not that Thellim is worried about that or anything, it's not like anybody here thinks that could possibly be anomalous somehow, or in any ways related to this being the kind of universe that has an impossibly bizarre lunar eclipse setup and terrible airplane food that people don't notice as strange -
She's getting off-track again. She's supposed to be conditioning right now on the assumption that this universe does have a consistent historical logic.
Thellim can't think of any arrangements for non-magical homosexual reproduction which would be evolutionarily stable over the long term, faced with competition from ongoing heterosexual reproduction. This could just be because there's a set of adaptations for stably trading kids that Thellim has failed to envision. But the more likely hypothesis - again going off premature-observation-cheating from Isabella mentioning relatively recent changes to sexual norms - is that the current situation is not in near-equilibrium evolutionarily. Give it a few centuries, and heterosexuality will become more common; the remaining male-male couples will be frantically competing to become one of the couples that has a thousand children apiece with the female-female couples; and everybody will be consciously desperate to have children.
Continuing that reasoning further: If 'dom' and 'sub' polarity isn't the effect of bipolar sexuality being destabilized by magical reproduction, the obvious next thought is that 'dom' and 'sub' are the causes of that destabilized bipolar sexuality.
Supposedly, Isabella's Earth doesn't put any care at all into preventing economically-adversarially-optimized superstimuli from being developed and released to the Network - has no concept of infohazards in general, apparently. And to a first approximation, Earth has nobody in charge of its globally important features, which is why the interdimensional alien visitor got picked up by one helpful psion and taken to a small apartment. So some porn-making corporation could have developed superstimuli versions of sexual polarity, icecreamizing male to 'dom' and female to 'sub' in ways that bound more tightly to sexual receptors than the original sexes; and then that meme spread like a detonating supernova through their whole civilization and entirely displaced masculinity/feminity. Without there being even the theoretical possibility of anybody who could stop to question whether that was a good or bad thing. And the new situation isn't evolutionarily stable, but Isabella's people can't let themselves think about that or where the new setup will trend or eventually stabilize, because then they'll go insane and start stabbing each other with kitchen knives.
So that - feels like Thellim's current hypothesis, more or less? Her guess in advance of observation? It's not a very polished hypothesis, but Thellim doubts she'll come up with a much better one for further thinking. The point here isn't to be correct, so much as to notice what she learned after finding out the correct answer - to notice which parts of Thellim's wrong background reasoning helped produce her no-doubt-wrong conclusion.
Of course, that's assuming the Network even contains an answer. Most of Thellim's guessing-probability is on "There literally isn't a historical reason, and people in this layer of reality are not noticing that their airport seat designs and sexual emotions aren't plausible equilibria of historical developments."