A sexual relationship that doesn't disallow sexual expression or affection with other partners. This may present itself in many forms and many kinds of lifestyles including but not limited to swinging, polyamory, polygamy, open relationships, or simply couples having an occasional -threesome. Yes, even if you romantically kiss only one other person while in a sexual relationship with with someone else, you are exhibiting a behavior that is clearly not monogamy.
We're mostly monogamous but sometimes we like to invite another woman into our bed, so I guess that makes us non-monogmists.
Every year we get crazy and allow each other one night of non-monogamy action.