The first thing I did was add the Loyal trait to all of my sims.
The first thing THEY did was immediately cheat on their partners, even ones who had never cheated before.
In my game, the Loyal trait seemed to guarantee that sims will DEFINITELY cheat...so, it's backwards and works the opposite of the way it should, just like everything else.
If anything, it encouraged cheating in my game and seemed to be more of a push to give them tense/guilty moodlets about cheating than anything else. It certainly didn't stop the behavior.
If you're not opposed to mods, use the "Committed" trait by Chingyu. THAT actually works and DOES stop sims from cheating.