It has been a long since Kyle accepted he would never get to mate. That's why, when Stan started courting him, he thought he was just being nice, not that he was head over heels in love with him.
Kenny doesn’t have much. He doesn’t have money, he doesn’t have a nice house, and he doesn’t have new or shiny things.
All he has is Butters.
So, when Butters develops a crush on Kevin, Kenny realices that being “nice” isn’t going to keep his world intact. He has to do what he has to do, and even if he feels guilty, he will never regret his actions.