I don't live in the US so thay isn't what I am basing my argument lol, I literally stated 18 was abritrary and said the issue was the teacher student relation where that is illegal in many US states or, more relevant to Japan, I am pretty sure illegal for students under 18 even if they are at the age of consent there. Even if you want to stick to non-teacher scenarios, in Japan, sexual relations where a minor is involved and the age gap is above 5 years is illegal anyways lol.
Buddy, 16-17 year olds back then had lower life expectancies, more responsibilities thrusted upon them from a young age and lived in times where they didn't have time to wait for 16-17 year olds to mature for all kinds of reasons. We now live in the most safest time historically where 16-17 year olds can go and have a full education without being expected to work a job without (for the average person in a 1st world country) having to worry about putting food on the table as their parents can cover that.
The world isn't like how it was for the majority of the past 2 million years or whatever so stop trying to apply those rules now 😂. According to you, Gotou should be the head of the household raising 3 kids with 2 wives because that is what happened for most of history lol.