Because literally their actions speak to another set of values than what you're describing.You go to an awful length to try to deduce what I think or what my values are. What does it matter what those are? This is about the manga and what the characters think and their values.
The parents are extremely family oriented because they have a huge family. Yet they tell their son to stay away from everything that leads to a family. How is that not hypocricy?
It also doesn't matter whether the parents dated in high school or not. It's not like the MC would have been around to witness that. He only witnessed the large family life, which does suggest the parents get along well. That's where he would have got his values, not from the parents' youth, when he didn't even exist yet.
I do admit, though, that the premise of "inappropriate relationship" = "dating" is stupid in my opinion, which is probably why I started to pay attention to this. Maybe if that wording hadn't been used, I would have never said anything about this whole issue, so I'm not without a bias.
You just come off as inventing a set of values that is not supported by their actions in order to label them as hypocrites.
You may not like it but that doesn't change what they have already indicated they believe in. All I added as a supporting argument was to point out how their, I emphasize, already established set of values, don't contradict their family situation.
If you just disagree with their values, then just say so. Don't try to give yourself moral high ground. What you're doing is imposing what you believe is an universal understanding of values onto other people and saying they are going against a value system they never subscribed to. That in fact they could even be morally deficient or irresponsible in some way just because they don't share your views. Sorry, what high chair were you speaking down from again?