Sims automatically perform certain romantic actions, even if they have a dislike for those actions. Because of this, relationship satisfaction drops very quickly. Example: A Sim automatically flirt...
Ideas
I played the family reunion event with a couple of my sims of different households and they flirted (and other romantic options) with everyone even father winter and all of them were married or had partners.
And after the event ended they would not go home they stayed at my lot for hours.
