I think Sehyun is thinking that it will be hard for the authorities to believe that he is rape given his reputation back then. Almost everyone in the bar knew him as a slut who will go with every guy. And if ever he will go to the police, they might dig up his background, instead of receiving help things may get worse. There are really a lot more disadvantages with that than advantages.
I think what's happening to him right now is the retribution from all the choices he made even from the start, and when he chose to tolerate the abusive Michael and when he chose to become a whore.
His situation was really pitiable, because he didn't have a choice but to go with Michael because Sehyun doesn't have anywhere to go and someone just offered him a home. But if he did not gave up on looking for a better life or any legal work, I am sure he won't end up being raped like right now.
If Sehyun was raped repeatedly why didn't he go to the police? The police would have done something about the matter. ╥﹏╥They always came inside of him so he could have given the police the semen samples and could have figured out the rapists easily. I really don't understand.