He's pretending to be a nice person because he knows they're being listened to. The sex in ch. 35 happened outside of the house and it's a possibility Sangwoo did it outside because he was suspecting that the house was bugged. What they said once they got inside was mainly Bum just telling Sangwoo he likes him. The following night Sangwoo probably knew Bum is too dependent on him to say no so he went for it to give off the impression that they're an actual couple. The following morning Bum said out loud that he liked it, giving off the impression that it was consensual.
Yeah, to the detective it probably came off that way because he knew the truth but it couldn't be used against Sangwoo in court since Bum said the following morning he liked it and even if the detective reported rape or assault, it wouldn't hold in court because they would need Bum to witness that was the case and he's too fucked in the head to betray Sangwoo now that he has the false impression that Sangwoo actually cares about him. Sangwoo is probably not trying to convince the detective that he's innocent, he's just careful to make sure he doesn't say anything that can be used to prosecute him if/when the detective goes to the police with all the material he's recorded.
Sangwoo is treating Bum nicely because he's prolly noticed the house has been bugged.