It's bad for partners to flirt with other people.
John and his girlfriend Sarah are having a drink at the bar when John goes to the bathroom.
Sarah wants to make conversation with someone while she waits for John to get back.
Sarah strikes up a friendly, casual conversation with the bartender.
Sarah begins saying sexually suggestive things to another patron seated at the bar.
John will feel secure in his relationship with Sarah, knowing she isn't flirting behind his back.
John will become jealous and angry because he thinks Sarah would rather be with someone else.