If there is something I learned reading this manga is japanese romance authors have very mysterious neural pathways and they won't write the obvious logical situation sometimes.
I have a feeling they just have experience. If I write about my dating life in 20's it'll be muuuuch more fucking messy than this, with broken bones, blood, cheating, bdsm, constant ambiguity of everything (including gender), a lot of alcohol, controlled substances use and a bit of psycho shit. This even with a pinch of heavy stuff (Yami family life) feels nice, fuzzy and almost warm like. Just teens, fucking confused, fucking up, but nothing bad really happens.
Also, peeps, stop trying to idolise characters who are supposed to be human. Humans are messy. Humans relationships doubly so. Hell, I think classical literature will give you aneurism if reading this heats you up so much xD