That's the thing though. Absolute monogamy like that never existed, the powerful men at the top had always kept mistresses when they could, and young, unmarried teens have always fucked around as much as they could. From the medieval era for example, we have tons of proof that noblemen and other rich men have fucked other women than their wives, a substantial number of people have been single their entire lives, cheating and extramarital sex of course existed, and while the authorities didn't like fornication, they barely cared to do much more than fine the people who did it.