What suspicions? I think she was suspicious in the sense that he's been acting weird, but i don't think she thought he was cheating on her. If she did, i don't think she would've thought it was a man, let alone SANGTAE since their last conversation was about how he needs to get over his "inferiority complex" towards Sangtae.
I never said she thought it was sangtae lmao I’m just saying she did have suspicions of cheating when he came to her late, it’s implied not really gone in depth. She now thinks he’s cheating or something because he had a dildo in his ass after he couldn’t get it up with her but has been able to in the past. Chill ya titties
"chill my titties"? Where did i come across "not chill"? It was a very mild response, don't read into things. I honestly don't know how you took offense to it. I don't agree that it's implied she thought he was cheating, she just thinks he's acting suspicious, that something is up. She doesn't say anything implying that she thinks that "what's going on" is that he's cheating, just that she thinks he's acting weird. If you're going to argue otherwise, please provide pages so i can see. Thinking "something's going on" with someone doesn't magically make you aware of exactly what it is, if it worked that way then the police wouldn't have to do any investigation as they'd immediately know the guy's a criminal as soon as they had suspicions he was.
The page where she ends up leaving the first time, she’s in a relationship. Everyone has suspicions in relationship. Cheating is a common one, and there’s like 1% it never went through her mind. I’ll find page numbers if I have to but I don’t wanna go through it. If you never been in a relationship with someone you might not trust you won’t understand. So honestly in the moment when she walks in on him with a dildo up his ass and he can’t get it up with her but he has a raging hard on now, it’s implied. Also it’s not that she doesn’t think it’s a kink of his bc she’s been with him before.
Honestly, i think it's because she caught him lying to her, a 2nd time, he had said he was sick so she came over and he showed up late. If you think about it, she catches him lying to her just now in the breakup scene, he said he was sick again so he can't get it up and she walks in on him full mast playing with his ass. I feel like the natural line of thought in her position would've been: "he lied to me and showed up late, weird"-> "he didn't get hard and just lied to me saying that he COULDN'T because he's sick? Again, weird..."-> "He couldn't get hard with me, but he's crazy turned on with a vibrator in his ass? That must be it, he likes anal play and that's why he has been lying to me and couldn't get it up". This'd explain her dialogue where she says "you could've been more honest with me (this implies she is irritated about the lying)... And told me you wanted to be the one to take it inside". She doesn't say anything about cheating, she mentions "honesty" and him wanting it in the ass. She thought something was up and because of this latest scene she misunderstood that "something's up" in her head to be "he's lying because he likes anal play".
I can't really say anything else, tbh? I mean, if you want to view it that way it's fine, but there really isn't anything hinting she thinks he's 'cheating' specifically. What is shown is that she sees him acting weird lately and you're looking at that and discerning specifics where none are given, which is fine, i just can't say i agree. The reason i disagree is because of the lack of evidence. I know what you mean, the scene where he comes back late and she says "this is suspicious, do anything suspicious like this again and i'm breaking up with you", which CAN be taken as that she thinks he's cheating, but since she never says so or even implies that that is what she thinks i find it weird to to assume the specific reason she's upset is "because i think he's cheating". ╮( ̄▽ ̄)╭

I just see people saying she "knew about Sangtae", which just isn't true. If she wants to break up with him over anal play, that's fine, it's just her sexual preference. Why stay with someone you aren't compatible with? Guys, the last conversation they had about Sangtae was at the beginning where she told him he needs to get over his "inferiority complex" towards Sangtae... As far as she knows, Jinwoo HATES Sangtae. Why would she assume a guy to?