Yeah I don't think them sleeping with each other was necessary, but I guess that's how the mangaka's mind works. He really thought that was the best way to make his point, I suppose. The point being, as I understand it, that feelings/attachment built up over years are more important than what many would refer to as "love", which he seems to portray as more of a temporary infatuation or something. I suppose he's trying to say the bonds are more important than carnal desires too. It actually reminds me of a study that I read at some point discussing how important building those bonds was to a lasting marriage, since at some point that initial "love" does fade away. I guess technically I read that study after I read this series, but yeah, seems related. And of course, that's why the title is "Seishun no After".
Note: Basically most of what I just wrote was trying to contextualize it a bit for people that do read the spoilers, and not necessarily a direct response beyond the first couple sentences.
