1) It is likely that all sexless people die young because they face the same issue with Hinase which is dealing with excessive amounts of female and male hormones which causes many complications and fatal if not treated. It's vague what being sexless or what the manga calls it "genderless people" have to deal with when it comes with endocrine issues and how it differs from intersex people. I never read all chapters so I don't know if intersex people were ever mentioned or even exist in the manga.
2) It was definitely a choice to make being genderless deadly so it is likely for a cause of urgency because Hinase might have never chosen a sex or a gender without this possible cause of death.
3) I'm not sure about what you mean by option. Do you mean choosing to remain without a certain sex or not identifying as girl or boy? Perhaps both. It's hard to determine what is realistic as everyone has their own definition to what is the norm or real. The author could have a very different idea on being realistic from being from Japan. To me I don't think it is realistic because it is fiction and uses its own logic. But it's genuine in its own way in demonstrating how transitioning may feel like and how faucets in identity or sexuality work.
Both but I think the latter works best in this story, i just found it interesting that the author never considered any characters of just not conforming to either gender (even with a binary sex!) I’m pretty sure enby folks live in Japan if I’m not mistaken (though I’m not sure what the differences are versus non-JPN folks on how they are presented) but I can see how people reading the story can be mislead, though I enjoyed it. I don’t think the author was malicious by any means, I just think their world-building has some merits of criticism lol.
I really like this manga but I’d love some discussion on this, can someone explain to me why the world-building has it so that sexless people die young? Was it to make Hinase’s choice more urgent? Because I feel like the themes of sex and gender still resonate if that was an option. Maybe it’s to be more realistic? But the fact that’s just the absence of any non-binary identity while choosing a sex seems so odd. I really so like this story for touching directly upon gender in such an interesting way but the author’s intentions here are confusing me.