The problem is sim emotions aren’t targeted. So if they’re in a flirty mood (and having recently flirted with a significant other, or even just being in the same room as a sim they have a flirty sentiment with makes them flirty) they will choose to autonomously flirt. With anyone and everyone. All the “loyal” trait does is make them feel bad about it.
When I’m playing a couple with high romance, I basically just don’t let them socialize with neighbors together. They either are alone together, or I send one away and let the other visit normally.
