Diane Lane's character represents the target audience for this film: a housewife with a good marriage who nevertheless perhaps feels that there's a little something missing in her marriage. Maybe she feels her husband takes her for granted, or she feels confined in her house and her role in life, etc. She fantasizes about what it would be like to be swept off her feet by a buff, younger man -- maybe even one with a foreign accent.
Of course the movie is unrealistic from beginning to end. First, few wife/mothers entering the early stages of middle age have Lane's body. Second, the odds of running into a French playboy are slim at best. Third, it's probably not going to end in murder, but will almost certainly end in disgrace, divorce, the loss of both family AND boyfriend.
So go ahead and fantasize, ladies, but if the movie convinces you not to act on the fantasy, it will have served some small purpose. And give me an honest answer: If you thought the French guy was sexy, didn't you also get the feeling, when Gere's character confronted him, that it was a meeting of a grownup and a boy?