Here's a idea for a new kinky trait: Cuckold.
For people unfamiliar with the term, a cuckold is the husband of an adulteress. It's usually an insult, but it's also a paraphillia -- there are guys who are turned on by watching their wives with other men.
A woman who cuckolds her husband is a "hotwife".
A woman whose husband cheats repeatedly on her is a "cuckquean", though that's an archaic word today. The rest of this article is going to use pronouns that assume that the cuckold is a man, and that the cheating partner is a woman, and that she cheats with other men. It's the archetypal case explored in classics like Lady Chatterley's Lover. That is not meant to exclude any other combinations.
So how would this work in KW? I see it both as a trait that can be assigned via the CAS and a reward trait that is earned. Model it after the Masochist trait. A Sim who does not start as a Masochist can earn the trait as a reward for being raped a sufficient number of times. In the same way, a Sim who catches his partner in woohoo with other men a sufficient number of times earns the Cuckold trait if he doesn't already have that trait.
A Cuckold Sim is like a Voyeur, but the positive effect of watching Woohoo only comes from watching his partner, not from just any Sim.
The Cuckold trait is different from the No Jealousy LTR that is already in the EA base game. Cuckold is a trait so you can construct a Sim household right at the beginning using a Cuckold partner. No Jealousy has to be purchased. And No Jealousy merely protects the Sim from suffering the effects of jealousy, while Cuckold acts to encourage kinky behavior.
So do we want a Hotwife trait as well? I'm not convinced of that. It seems like the Woohooer or Slut traits cover that already.
The "Encourage..." social interaction could be extended to include a new options available only to cuckolds and their partners. The cuckold could encourage his partner to woohoo with another nearby Sim. Or the cuckold's partner could encourage the cuckold to watch her woohoo.
Consider also blocking the "Join Woohoo" option when the cuckold is watching his partner. The cuckold enjoys sex, of course, and might do a threesome with his neighbors, for example. But not when he's watching his partner. That would just ruin it.
Should a cuckold get hornier watching his partner, or should he be satisfied? I'm leaning to the idea that he gets hornier, and then looks to his partner for satisfaction once she is finished with her special friend. She's going to be busy. Better send the kids to their friends for a sleepover -- this is going to take a while!
A final consideration is what happens if the Cuckold and the Hotwife split up? Does that change the trait? I don't think so. He'll just look for a new partner who is willing to indulge in his strange appetite to watch her with other men.