
More than an age gap, if you want, is the difference in both life contexts. Also, teenagers and young people in general (including 18, 19 and technically speaking, even until 25 y o) are vulnerable not just in the law, but in every other aspect. Life experiences are super important in their development and this relationships with older people don't make any good (I'm not talking about a 3-5 y o difference, but a bigger one).
Anyway, it's not like you can't enjoy this type of stories if they are well written and the art is beautiful, but you have to be critic of them and not justify them.

Though I agree with you the problem is that in this situation that’s unrealistic. This is a unrealistic narrative where there is no way that’s happening, in some other mangas you can see that the adult takes advantage of being an adult though. However, in this one The adult one doesn’t have more power than the other.

I totally agree with you. I won't excuse myself because I keep reading this type of manga just because I find the art to be pleasant or the story seems interesting in some way or, as you say, the situation is way too unrealistic.
However, the moment this takes a wrong(er) turn I'll immediately drop as I have done many times before.

Thank you for the update!
But. Blaming the victim of abuse is not excusable. I know it's fiction but most of the times, it reflects reality and drives people to think that being possessive is romantic and it will be forgiven in the end and that's a HUGE NOPE for me. It's not Jaeun's fault if Jihan doesn't trust him. Yes, Jihan realizes that his behaviour is wrong (and I'm glad), pero me cagüen la leche, this is exactly how the cicle of abuse works... I'll keep reading this because I hope it changes.
I really can't stand cheating. It reaaaaally gives me bad feelings when reading about it but here I go prepared to suffer again.