I won't be negative here as "Kink Shaming" is never a good thing. One of the problems with the Nice Time for Roo route is based on the fact that unless the lover starts actively refusing to sleep with you and/or degrades you when you catch them cheating than it won't really have an impact considering many of your lovers already sleep around (Cait, Arona, Brint, Berwyn, Quintillus) as does your Character. I think the only lovers that could cheat on you would be Brienne, the Cat Harem and maybe Gwyn (dunno how your relationship with Gwyn actually is.) And I don't expect Wsan to ever have Brienne cheat on the PC, she won't take any additional lovers.
The Closest you would ever get is a Companion or Lover who could cheat on you but will not be able to steal your lovers (as this steps on the toes of many different creators.) And you would either need to create them yourself and see if it passes the review or cross your fingers to see if another creator does it for you. I would note that if you write them up they won't be a companion as you need to be "within the circle" so to speak to get the honor of creating a companion.
Actually, the mechanics of how to do it properly are relatively simple, though it would necessitate the introduction of an additional character, so as not to piss off fans of already existing ones.
Just introduce a very "vanilla" character who wants to enter a very traditional, milquetoast relationship with the protagonist, and attach the aforementioned "Loyalty Meter" to said character. The more it goes down, the more blatantly she/he (probably she) starts cheating on you, whether with the townsfolk, random people, or your party members. If said Loyalty goes down low enough, instead of just cheating she starts openly trying to humiliate you in the process (think Sera from TiTS), including situations where you go back to the inn to switch out party members only to find out that you can't since said party member is railing/being railed by your "waifu". Getting pregnant with someone else's child or including you in the sex would be the low points, I think.
Hell, you could also do the opposite. If her Loyalty goes TOO HIGH, she'll start entering yandere mode, with all the wonderful scenes that would entail.
Something like this:
- Normal Loyalty, No Corruption - Vanilla Content
- High Loyalty, No Corruption - Voyeur Content
- Very High Loyalty, No Corruption - Cuckquean Content
- High Loyalty, Moderate Corruption - Jealousy Content
- Very High Loyalty, High Corruption - Yandere Content
On the other hand:
- Low Loyalty, No Corruption - Cheating Content,
- Very Low Loyalty, No Corruption - Cheating in the Open Content
- Low Loyalty, Moderate Corruption - Nice Time for Roo Content
- Very Low Loyalty, High Corruption - Humiliation/Forced Content
Such a character would also be nice to underline the "corruption" aspect of the game. As it is right not, nearly everyone is a sex pervert from the get-go.