Responses
Exactly, this is what happened.
I think it was when Iwaki was still denying his feelings and they were living apart from each other
No, they were already living together but Iwaki was still keeping some distance. Iwaki (thanks to Katou's friends) discovered it later in the story but he accepted it as a mistake from Katou's past.
I know he didn’t “technically” cheat but when did he go to the orgy? Please tell me it was at that time in the story where they had JUST started living together and didn’t talk much