Is it really an affair if it never became sexual?
Any relationship that takes the place of what should be given to your partner—either physically or emotionally—should be considered an affair. It steals what was promised to them and gives it someone else. Just because the investment is emotional rather than physical doesn't mean it is less serious. In fact, an emotional affair is usually more damaging that one that is only sexual and it is much more difficult to end. If affair involves both emotional and sexual connections, it becomes even more entangling.
Even online affairs that never cross sexual boundaries will likely cause a strong reaction by your partner because he/she feels betrayed. Any word, feeling, touch, or desire that was promised exclusively to your partner but is given to someone else in a relationship, whether casual or serious, falls into the category of an affair.