I've not had any sims breaking up or even coming close to cheating since that is set to -100% chance of flirts and affairs outside of marriages unless I directed them to start cheating and the chances of them getting caught in the cheating is also -100% which means nobody who did happen to cheat would ever get found out.
That's now how it used to be playing Modless though as that used to be one giant soap opera and it meant sims getting caught in bad relationships or flirting with total strangers and being mismatched with the wrong partners in romances.That drove me to start using a Modded game to get control over that on a town level.