I don't get his state of mind at all. This is straight-up hentai logic for me. He loved a girl for years, never told her, set up some made-up barrier of getting into the same school, and when he failed in that, just one event, not even trying to confess, he jumped in bed with the first person who offered. To me, this really puts in question if he was even in love, or just wanted to "catch up to her".
My main issue is still me not caring about those events that have no real influence on the main plot. The fact they dated is probably enough, and no matter what happened, it still won't make me root for the girl who backstabbed her best friend.