Sims characters being unfaithful

Why can the Sims character flirt with someone that is not their designated spouse/person they're dating most people dont want their character to do this so it should only happen when you do it manually and never the Sims own decision. This way if you want your character to cheat they are free to do so but youre still in control and dont have to keep an eye out for it and shut it down when you see it constantly.