I don't really understand the story Can someone explain it to me?

A younger boy falls in love with an older man, which happens to be his father's friend. And also his father is the first love the older man
They somehow start dating with all the conflict and usual problems but at the end, the boy finds out that his lover had loved his father before and gets all worked up. Thinking that he was just a replacement for the man
So he breaks up with him and starts a new with someone else
And the older man, well he leads a sadder life~¬~
I hope i explained well enough °^°