Sometimes when you're in a one-sided love for a long time, it can be imprinted so deeply into your mind that even when your feelings change, your brain can't catch up and therefore makes you think you still haven't moved on. Kinda like making your one-sided love part of your identity. I think Koutarou probably already developed feelings for Gen. Notice that after found out he was nowhere near disappointed that it wasn't Kanade he did it with. If he was truly still deeply in love with Kanade, it would've been the opposite. Instead he was quite happy that it was Gen and kept on thinking about him after that.
I think this is it, this is the feeling i got reading this
I think it’s infatuation for Kanade, of course he still loves him in a way.
the way he moves on way too fast is just aint it, it feels like he start chasing after gen just bcs he knows damn well he cant get w/ his brother, this sucks.