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.

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.