Assume you don't want to be with someone who cheats. If you tell your SO "if you cheat on me I will dump you," and then they cheat on you, they might actually regret it and consider it a "mistake" and still want to be with you, and because they don't want to be dumped, the may choose not to tell you that they cheated. If you don't tell your SO "if you cheat on me I will dump you," and then they cheat on you, they might tell you to try and make themselves feel better/keep honesty open/etc. However, in this case, you have been presented with objective truth (your SO cheated on you) and can dump them accordingly. Remember that the goal is to base all your actions off of objective truth to arrive at the correct outcome.