If I'm not mistaken, to even get the cuck dialogues, you have to enable it on chapter 10 during Ian x Lena sex scene. If Lena fucked some other guys, she will say something along the lines of, "Ohhh, this is one of the best cocks I've ever had."I think the game fails to differentiate between cuck, corruption and cheating. I have no problems with Holly becoming slutty, I like "free spirited" girls. Same with Lena, she can fuck anyone she wants, I will not complain about Cindy fucking Axel, because she is also cheating on her BF with her. Ian is fucking anything that moves, and in my case I dont care if some chick I am not exclusive with is fucking someone else, I just dont want the cuck dialog.
Then you get to choose a response as Ian. The very obvious "One of the best...?" is the cuck choice. If you don't pick this, all the subsequent cuck dialogues don't trigger.
This game has two protagonists with incredible flexibility. Almost all complaints about how the characters act are self inflicted. Obviously, you get a bit less control over the NPCs, but even they are responding more or less naturally (according to their personality) to your decisions.