Cheating (being involved with someone else, with secrecy and deception) is a destructive way to be nonmonogamous. On the other hand, swinging and polyamory (types of responsible nonmonogamy based on the knowledge and consent of everyone involved) are viable choices for some people.
Jen: I don't want sex if you don't want to be monogamous.
Brad: Sure- monogamy's my one true love, honey... Besides you of course.