Someone needs to tell seto there is no such thing as normal just what we (and society) perceive as outsiders looking at only the surface level of other people relationships, and it's perfectly acceptable to not not be in one at his age.
Also that male treacher is scum
I disagree with you, "normal" is a hidden rule and acts like guidelines.
“
The code is more what you'd call 'guidelines' than actual rules.” – Barbossa, Pirates of the Caribbean
You need "normal" for a society to function, this applies to other animals as well, not exclusively to humans. You can check throughout human history, that there were times "normal" so loose, that the strong ate the weak brutally.
It is impossible to build a society that satisfies everyone, we humans are not that capable on that level yet. Cases like Seto are abnormal and need to be handled by specialists aka therapists, psychologists, etc
Humans are social animals and yes, they can live alone as well but the burden is very heavy and lonely and only the strong can walk. That is why Seto wants to be accepted.