In an Ensign article, Kenneth Matheson speaks about spiritual and emotional fidelity.
Emotional infidelity…occurs when emotions and thoughts are focused on someone other than a spouse….Emotional infidelity doesn’t usually happen suddenly; rather, it occurs gradually—often imperceptibly at first. This is one reason why those involved often feel innocent of any wrongdoing….Relationships with others of the opposite sex are not in and of themselves a problem or a fracture of fidelity. In fact, many of our meaningful relationships with neighbors, Church friends, co-workers, and others have a balanced and important place in our lives. However, there is a danger zone that people may cross into if they are not watchful.
This strikes me as perfectly true and wise. However, Matheson makes another comment, which I am not quite so sure about:
A marriage can be placed in a precarious situation when one spouse forms a relationship with someone outside the marriage and begins to choose the company of that person or frequently shares personal information with that person rather than with a spouse. Furthermore, the problem can occur with either husband or wife. “Jane” could just as easily be “John.”