Maybe if you consider and understand the reasons they said on why they are into shinwoo and shinwoo's reasons too, maybe you wouldn't be saying this.
I understand your irritations, i also don't like MC uke who cheats on their bf but after reading/seeing their reasons, i can't help but just understand why things are like that in their situation.
I hate shinwoo for the misunderstanding he had to sebin and i also hate taesoo for the thing he did in the past to shinwoo. Taesoo is the reason why shinwoo has BDSM kinks.
Nevertheless, the ending is good enough to forget and move on to the hate some readers like you and me felt and begin to appreciate the story.
THIS FUCKER FUCKS TAESOO THEN HAS THE FUCKING AUDACITY TO GO TO SEBINS HOUSE AND TELL THE GIRL HE'S SEBINS BOYFRIEND?? LIKE BITCH I-
ANYWAYS SEBIN DESERVES WAY BETTER THAN SHITWOOS NASTY ASS LIKE. OH AND HE ALSO GETS MAD AT SEBIN IF HE'S WITH ANY OTHER PERSON THAT ISN'T HIM LIKE YET HE GOES AROUND FUCKING TAESOO. ALSO HE JUST LEADS TAESOO ON. THERE ARE NO GOOD QUALITY'S ABOUT SHINWOO HE'S PURE FUCKING SCUM AND BOTH SEBIN AND TAESOO ARE STUPID FOR WANTING TO BE WITH HIM
(I wrote this right after reading this so I apologise for grammar and etc but like just I hate shinwoo so FUCKING much like dude belongs in a circus