Jaemin assumes that he only got confessed to so he wouldn’t get the top score because he’d be too busy dating and stuff, so he goes along with it. He doesn’t know the feelings are real. The miscommunication gets addressed later and they do eventually start dating for real, lmk if you want anymore details (quite a lot happens in the middle)
Basically after Jaemin tells him that he just lied and faked his confession, Seungpyo gets super upset and mad and says ok now we have to sleep together and Jaemin just goes along with it because he feels guilty since he assumed that Seungpyo was faking his confession. They both prepare for it and Jaemin goes over to Seungpyo’s house when his brother isn’t home and mid sex, Seungpyo folds and says he’s gonna make Jaemin feel good because he sees how much Jaemins enduring to make it up to him. They end up properly confessing after and get together for real because Jaemin understands that he truly does love Seungpyo

Novel reader, pls spoil for me