Well the problem with that is that youre trying to apply real life norms to this weird dystopian world where might makes right and so far, Zahra is also a believer of that and she believes that the MC is the very best with a touch of actual love for the guy
Sure she *could* end up falling in love with a "beta" but at the moment, we have zero reasons to believe it
It's an interesting question: are there core moral concepts which are univerally applicable for all humans, or are all moral concepts just social constructs?
But in this particular case, this is not a completely alien civilization, like the Aztecs before the arrival of the Spaniards. The world of this game is based on a combination of contemporary Western culture, some contemporary Egyptian culture and some ancient Egyptian culture. All of that twisted by a nuclear holocaust. Still I think there are plenty of reasons to apply modern real life norms to this dystopian world. Besides that, both the dev and the players are rooted in the same contemporary world, and therefore create or experience the story in a way that makes sense to them.
With regards to Zahra, I think she does love the MC and is turned on by the crazy stuff that he does to the other girls. And it may well be that she enjoys herself with the other harem members while the MC is away, which is totally fine by me.
To be fully clear, I also don't believe she'd cheat on the MC with some other guy.