I think you may have left out "because it is natural". Numerous scientific studies and human history has shown that people are not inherently monogamous. What is completely "unnatural" is to say you are going to commit yourself to one person for the rest of your life when you don't know if your partner is going to turn into a serial killer next month. Doing something like "getting married" doesn't suddenly stop your attraction to other people. We'd like to think it is true, but the worst lies are the ones we tell ourselves. It is quite remarkable that a person can say with a straight face, that they will go against their own nature for the rest of time. Good luck with that!