If you hate it don't talk about it. It's been talked about and discussed many times. If you hate it so much don't think about it or read the novel.
I'm tired of people saying they hate the relationship without trying to understand what is happening and how the relationship plays out. She is of legal age when they start their actual relationship, yes, she's 18 when they take in the relationship.
Plus, people act as if the whole show is ruined because of this relationship. The show itself doesn't really focus on the relationship it focuses on empathy, which is what it was meant for.
You think this is bad read Otoyomegatari. Also, just so you know, child marriages (not talking about Violet Evergarden) was quite common in many cultures across history. So, Otoyomegatari wasn't even that weird.
My point is, don't like the relationship, don't talk about it. Like the relationship, great.