Well, depending on how you play the game there's cheating he doesn't know about. Martin, Marcus and the beach orgy that follows, two of the three variants on the spa-or-beach scenes with Drake or Drake and Troy, the doctor, and I think I'm forgetting one.
But I also think the thing you're wishing for is actually fundamental to the game and has been all along. Anne's disrespect for/lack of interest in the MC grows with each update. I've posted roughly a zillion words about this in the game's thread and won't repeat them all here. But compare the early days, in which Anne and the MC have a lot of sex and exchange a lot of romantic words, to the latter days in which Anne and the MC almost never have sex (with each other) and she hardly ever tells him she loves him. She stopped recounting or even being honest about her escapades a while ago (and he's never been honest about his, even after she dresses him down about it). She's expressed her resentment at being pushed into sex for his entertainment more than once. She complained about Kenny's spycams. If the MC cheats on her she confronts him on up to four different occasions (five if you get the scene where she complains to Troy). The first time she has sex with Dre she says that she would've preferred that the MC not join in (if he did), and she makes it fairly clear that his imminent weekend return is going to exclude the MC. How long has it been since the MC's had any influence over Anne's extracurricular sex life? By chapter 10 she fucks who she wants and doesn't much care what the MC thinks.
Note that the original name for the game was "The Cuckold" and that's still the name of the save directory. From the very start it's been a Pandora's Box/"be careful what you wish for" game that got tangled up in offering other ways to play it, but by the latest pre-revamp chapters it's very obviously driving a serious wedge between the MC and Anne. I have no idea what the outcome of the revamp will entail, but I can't imagine the core ethos of the game is going to change.
I don't think actual NTR was ever particularly considered as part of the game, and in fact there's almost none until the very end of the last chapter and the Ryan incident. Previous to that, the only times that there's even a hint of it are on the beach when Anne and Mike visit the bathroom, at the swinger club (depending on how you play it), and any time the MC goes home for lunch and looks around for a mysteriously missing Anne, wondering where she is and whether or not she's having sex. Which, on two occasions (depending on your choices), she is.