So what happened in High School? I am confused.

Dohyun was a jerk in High School who, through his actions, told Sungho that they could either sleep together or break up. Sungho didn't want to break up, but he also didn't really like sleeping with Dohyun. However, he went along with Dohyun and slept with him because he liked Dohyun and din't want to be alone. Ten years later, Sungho kidnaps Dohyun and rapes him for "revenge."