I get this is only the first part of the novel, but damn, way to flub the ending in pretty much every way. Let's go down the list.
1) The point of the plan was to emphasize your status as victims of your partner's cheating. Telling everyone you're hooking up with your cheating ex's former partner the instant you break up really doesn't make you look like an innocent victim.
2) You ruined or at least damaged your master plan by casting doubt on the idea that you were innocent victims...and you don't even get together for reals. So now it looks like you were mutually cheating on each other and broke up when the filthy thrill was gone.
3) Weird, gross fixation on Touko's virginity. Seriously, who cares, ew.
4) Cuckold guy gets to keep his cheating girlfriend. Yay I guess?
5) Seriously, zero consequences for Karen and Fuckboi? She walks off and he just stands there and that's all? Come on, give me something to show they're getting their just desserts.
Very disappointing, 3/10.