Mods / Traits: No Random NPC Romance by lazarusinashes – TSR.
This mod prevents the game from generating romantic relationships between NPCs. In my last save game, my Sim was engaged to a roommate. This same roommate went to class one day. Later in the day, I added her to my family for something routine and, to my dismay, I found out that she had cheated on my Sim when an NPC introduced himself to her. It completely derailed my story! Since then, I have been deadset on finding whatever was responsible for that.
