You don't just have two choices. Not even sex is binary, much less gender.
I mean I know Japan's birth rate is declining, but it's actually not that bad. It doesn't need to reinforce a hetero-normal and binary society anymore than it already is.
The conflict here is just a red herring to a much bigger conflict, albeit only possible given the premise, but it's actually not that far off from real life. People do get forced into either one of 2 boxes, and it's claustrophobic just to think about.
I don't stand for raising kids genderless and asexual, but do teach them to be critical about it.
You know, for a society so close-minded, I'm glad there are authors who won't stop defying the enforced norms, and I hope this one follows a similar path and give us a true resolution, and not just "boy or girl, choose".