LOYALTY/CHEATING TOOLS AND GAMEPLAY
Sims have a new self-interaction to "Disable Attraction" - This disables Attraction to any sim that this sim isn't already involved with. Combining the Ask to Just Be Friends interaction (zeros Romance Bars) and this self-interaction should keep sims from flirting outside of their established relationships.
Taken Sims (sims with official romantic relationships) can be a Turn-On for functional homewrecker deviant Sims.
Polycules
Sims can form relationships with more than one sim in base game with the new Romantic Boundaries, but this won't stop them from potentially flirting outside of their relationships. Their partners won't care if they are set to allow this. The "Disable Attraction" self-interaction will help with this, and can be used on any sims within the group who don't have or plan to have lovers/partners outside of it.
Closed Relationships
Disabling Attraction on every sim in a relationship, plus using Ask to be Just Friends on ex-partners/love interests (just to be safe) will prevent flirting outside the relationship from causing autonomous drama and damage to relationships.
Open Relationships
You could "Enable Attraction" on a previously closed relationship to allow partners to seek additional romantic/woohoo partners, or never close it via "Disable Attraction" to begin with if you prefer.
Noncommittal Sims
Maybe you want one (or more) sims in the relationship to be loyal to each other, and one (or more) be prone to cheating. Simply "Disable Attraction" and clear old relationships via Ask to Be Just Friends for the loyal sim(s) and "Enable Attraction" and keep any extra romances for the Noncommittal sim(s) in the relationship.
Affair Partners
Sims can choose to be the homewrecking type with the BG Villainous Valentine aspiration, and now they have the option to be Turned-On by Taken sims. I'd keep any romances they build with sims, even when those sims become involved, as it could lead to potential drama down the line later.
Aro/Ace Sims
You could "Disable Attraction" to turn off the Attraction system and prevent autonomous flirting even when single. You might want it on to find compatible partners while playing these sims, but you can always "Enable Attraction" whenever you wish to use it. It might make for more believable autonomy if disabled when rotated away from Aro/Ace sims too.