I needed a night out. After months of cutting pancakes into hearts, Googling stain removal hacks, and playing “dragon goes through airport security” with Sisyphean regularity, downing elderflower cocktails at a lavish, kid-free wedding offered a reprieve. I laughed with abandon, danced with as much abandon as a self-monitor like me can manage, and generally reveled in release from responsibility. Until, that is, I saw a married friend flirting with a young guest.

How could he? How dare he?! With complete and categorical certainty that cheating could never be justified, I intervened. His response — not just angry but affronted — took me aback, but still, I didn’t doubt that I was in the right.

TV, movies and pop songs had told me that cheaters are bad people: selfish, callous, and depraved at their core. Sure, some songs acknowledged that the tendency to stray could be catalyzed by alcohol (Kid Rock’s “Picture”) or unmet needs (Amy Winehouse’s “You Know I’m No Good”), and directors sometimes even encouraged me to feel affection for a character who’d landed in the wrong bed (“Brokeback Mountain”). But the endings taught me that punishment is always warranted, like when Diane Lane’s character ruined not just her own life in “Unfaithful,” but also the lives of Richard Gere’s and Oliver Martinez’s characters. I learned my lesson, as Madame Bovary swallowed arsenic, Anna Karenina jumped in front of a train, and Edna Pontellier drowned.