Responses
Yeah, it sounds like Chulsoo and Ye-Eun didn't actually have sex at any point during her relationship with Jiha. She'd said no up until that argument, then he tempted her into it by saying she could essentially beat the hell out of him to relieve stress. I don't know if there was even foreplay, TBH, it felt more like she just lay into him with the cane until she felt better. Which still pushes the line when it comes to cheating, but given Jiha made it abundantly clear he would never do anything like that with her, I get why she'd do it. And she DID try to tell him that it wasn't right to let it go on like that, too.
Ok so fair warning, this is gonna be a long one. With that said, here we go:
First of all, I want to say that I don't think any character is a "bad person", just made some bad choices and acted either immature or not very well. Though it doesn't justify anything anyone did, I'm just not gonna write anyone off as "good" or "bad" because of their poor, often misguided behavior. But I kind of understand where everyone was coming from. Chulsoo is bring all this shit from the past back up because he is jealous that Ye-eun was staying at Jiha's and wanted to hurt him by telling him all this. In the flashback, Ye-eun wasn't getting sexually satisfied with Jiha and she was afraid of him finding out because she didn't want to cause friction in their relationship. It seem that she and Chulsoo had experimented with S&M before shit hit the fan. Idk when that started, but I think it was before she and Jiha got together? Also, it seems that she and Chulsoo didn't actually have sex(of any kind) while she and Jiha were dating. I think at most it was just some foreplay. I know that is kind of sexual, but personally I don't see that as "sex". See, as a dom, played a role, and Chulsoo also played a role. I see it more as an extension of acting. Like how people "kiss" in a movie or a play or on TV; yes, there is physical contact that in other cases has deeper meaning, but in these kinds of cases, it's not really more than roleplaying. Also, had Jiha actually listened before losing his shit, he would've known that she was ending it with Chulsoo. But nope. He had to lose his shit. From his POV, it looked like she had cheated and was trying to cover it up. So it's understandable why he would be upset. HOWEVER, how he reacted went too far. He NEVER should've gotten physical with Ye-eun. That was really uncalled for. ALL of this would've been resolved with just this one thing; COMMUNICATION! FOR FUCKS SAKE! And that is the end of my rant