Kim Dan has been drugged. He's out of his mind right now.
I think he's just desperate for the sex because of the drugs. He's just saying anything his lizard brain thinks will keep the sex going. There's no filter, it's verbal diarrhea.
He may feel the beginnings of infatuation for JJK, but him being In Love all of a sudden makes absolutely zero sense.
Therefore, I believe Kim Dan didn't really mean the "I Love You" during drugged sex.
Lmao how is this shounen ai? Not that I'm complaining lolol