@SERVERFRA You're very lucky. The random/obsessive cheating has been a long standing problem in my game. It started back when I was playing the vanilla game, and is actually what drove me to start using mods to begin with.
I never used mods in Sims 2 or Sims 3, but I have found them to be an absolute necessity in Sims 4 because of the way autonomy (and other things) got coded in this game.
My story would have been destroyed years ago if I hadn't started using mods to stop the madness.
I'm happy to hear your experience with the vanilla game has been different than mine was, and I hope that continues to be the case.
For any others who are still struggling to find a way to stop the cheating that's so strongly pushed and enforced in this game (for whatever reason), Chingyu's Friendzone mod is also fantastic and works beautifully. It's been a game saver for me.
Happy simming, all! :)