From The Movie Snob:

Unfaithful. For once, I am stumped trying to give a movie a grade. You are probably familiar with the basic plot. Diane Lane is married to Richard Gere, a mild-mannered and decent sort of guy. They have an 8-year-old son and a nice house in the 'burbs. Then, Diane has a chance meeting in New York City with a good-looking young Frenchman, a rare-book dealer with an apartment full of books and sculpture. Before you know it, they're involved in a torrid affair despite the lack of any evidence of any reasons Diane might be unhappy with her marriage or her life. On the one hand, the performances are good, and the story is well-told. On the other, it is hard, if not impossible, to sympathize with the main character and her complete selfishness. That's about all I can tell you about it. You were warned.