To cut a long story short, I had the chapters translated. The story only gets more degenerate. She doesn't stop doing anything with anyone, and only adds to her activities with time. Anyone saying otherwise is coping/lying:
1.
She's still sleeping around. She says she doesn't want to sleep with Sanada-senpai anymore because she got bored of him (Chapter 69) and suddenly everyone thinks she's turning over a new leaf.
2. She's made it clear the only thing that'll (allegedly) stop her from sleeping around is her boyfriend's say so. They're still not dating, and MC took her message to heart. But he's not ok with it now, and isn't ok with it at any point in the future.
3.
She's already sat him down to watch her having sex with other men, and then volunteered him for a threesome(s) (Chapter 69). MC seems to go along with everything she does for the same reason men go along with "Open relationships" generally: She's going to sleep around either way, and he doesn't want to "lose" her.
4. In Chapter 69, she tells Sanada-senpai she's not looking to add to her circle of sex friends. He then says he'll do anything she wants sexually so she'd take him. At that point she proposes the threesome(s): It was her idea. MC was never consulted about this: She's likely doing it with or without him. Sanada-senpai only tells him about the threesome(s) after the fact.
5. In Chapter 70, she's taking them both on a date so the two of them can get along better, because "You can't have a fun threesome if you you're not good friends". Later on, tries to take erotic pictures in a picture booth with MC, and then when Senpai showed up, she invites him to join in.
All the copers have at this point is literal ignorance of future events and the author promising a "happy ending" (while ignoring how fucked up the author and story are).