You're right to be cautious when you hear these words: "I'm telling you, we're just friends."
Good people in good marriages are having affairs. The workplace and the Internet have become fertile breeding grounds for "friendships" that can slowly and insidiously turn into love affairs. Yet you can protect your relationship from emotional or sexual betrayal by recognizing the red flags that mark the stages of slipping into an improper, dangerous intimacy that can threaten your marriage.