This actually gets to the reason I think sharing Zahra doesn't make sense. She, by the character of her we have seen, wouldn't be into being with another man by choice, ever. Meaning that, as you say, the MC would be basically making her do it, no different than Ain or Emilia. That would imply that he doesn't see her any differently than he does those two. However, we know the MC does see her differently. It is one thing to share a slave, or even a daughter with a good friend, it is another to share your wife against her will.
Now, I hold to the principal of the "death of the author," meaning that it doesn't matter what the author says outside the work, it matters what we see.
Hildegardt, made a good point in relations to this. The developer's words, and even some of the off-hand comments in the library which largely depend on how they are read, seem at odds with what we see. In the game we see a culture where "might makes right" is the highest ethic. Sexual liberation and other things don't really factor in, and in fact, clash with that ethic. It is similar to trying to be a pacifist, you are allowed to be a pacifist because other good men are not and protect you from the violence of evil men.
I cannot fathom a single case were sharing Zahra makes sense give the dynamic we see in game, when the MC has access to a daughter and a slave that could be used in such a commodity fashion. Letting the brother-in-law sleep with Zahra makes almost no sense at all, that character, as far as I can see exists to be cucked. Even if a stronger character than the brother-in-law were to arrive and letting them have a sexual action with Zahra was "expected," the ethic of might making right would still overrule that and the MC would be right in denying that expectation and would be, I would think, held in higher standing for doing so.
If might makes right then sexual liberation does not exist, sex is at the whim of the strongest man at the time. A strong man keeping his women for himself should be just as, if not more, highly regarded as a man who uses his women to get ahead because he is not strong himself and trades them to get strong men to aid him.
Also, jealousy is not a cultural quality, it is a human one. Just like greed, gluttony, lust, and any other vice. The idea that Zeta has "moved beyond" jealousy in regard to sex is to ignore human nature. One could say that there is a sense to the MC being willing to share a slave, she could be seen as a commodity to some players, but letting another man have sex with a wife is different by its very nature. That is, if she was less than willing, and her being willing is dangerously close to her cucking her husband since it tacitly admits that he does not satisfy her sufficiently. It devalues her as a person and a partner, seeing her as no different than the slave girl, those are things we have seen the MC does not think. He values her as his wife and partner.
The brother-in-law is a weak man, and so his jealousy is irrelevant in Zeta culture perhaps, and it would be so for any other weaker man. A strong man's jealousy would trump the culture even if the culture was not one were jealousy was common. That aside, however, you still stand on the rocky foundation of a single strong man being able to upend the system.
From a real-world anthropological stance as well, even in cultures that were more sexual open, sexual restraint was still seen as an ethic. In Greek culture Athena, Artemis, and Hestia were virgins and it was the ultimate embodiment of female power. No man could take their virginity and they would only give it to a worthy man, and there wasn't one. "Sexual liberation" has devalued women. Men acted like cads, and powerful, worthwhile women called them to higher standards and men would rise to them. Until recent history and anyone with eyes should be able to see where that has led the state of affairs between the sexes.
All that to say this: the Zeta we see is not a culture of sexual freedom, sexual fluidity, lack of jealousy, or sexual commodity. It is one where "might makes right" and the strongest dictate the terms of all sexual encounters. Might making right is the only actual ethic we have seen continually referenced and play out in game. The queen is stronger than the prisoners and forces them to be whipped, burned, and used in the street. The guard in the alleyway is stronger than the slave girl and forces her to please him. The MC Stalker is stronger than the man in the street and prevents him from beating his slave. The MC is stronger than the brother-in-law and can take his women for himself. The MC making the raider girl give the other raider a blowjob was not because of sexual liberty but as way of devaluing her. The MC letting his daughter or slave give Igor a blowjob was not out of sexual liberty but out friendship and not valuing the women as anything more than he might a "glass of the good wine."
The best display of this ethic in two forms comes from the interaction between the MC and the queen. As the player we can acquiesce to her political strength or assert our physical strength. These two forms of might clashing is, I think, the best encapsulation of Zetan culture we see in the game.
I hope you have enjoyed another JoeTheMC essay...