It is rude to spy on your partner.
Kevin spies on Lucy because he thinks she's up to something.
Kevin wants to learn what Lucy's up to.
Kevin stops spying and instead just asks Lucy what's going on.
Kevin keeps up the spying through a detective, and he reports that she's seen with a bunch of attractive men.
Lucy tells Kevin that she got a new job as a photographer and her clients are a bunch of male models.
Kevin decides Lucy's cheating on him and divorces her right away.