I think it's a PTSD flashback, did you see him bound and bleeding from his head while his mother rode him? She brutally raped him. Judging from the dialogue, she did it on purpose to "kill him without really killing him". She wanted to break him, so she hit him over the head, tied him up and raped him.
Yeah I saw that part. Honestly the entire situation is disturbing and complicated. What sangwoo’s mother did to him was sick but it seems that she was mentally ill. Now bc of that sangwoo has also come twisted himself and committed countless murders. It seems like his father couldn’t handle the fact that sangwoo’s mother was sick and became angry and abusive bc of it. None of this excuses the things that any of the characters have done but I still think it’s important to remember that no one in this manga is really in a stable state of mind. Thanks for reminding me about the flashback. It put things into a better perspective for me!
I’m still so fucking disturbed that he called for his mom during sex （￣へ￣）