Many couple's I've worked with, use the emotional or sexual affair as a transformative relationship tool. They do this by learning by learning to communiate during corrective-bonding events at the attachment level of their relationship.
Healthy couples also use their affiars as a starting point for learning how to effectively communicate, listen-for and meet each others' core-relationship needs through systematic action planning and follow through. Learning how to affair proof the relationship through effective boundary setting can also make the relationship much stronger than it ever was before the affair. The opposite of an affair is meeting each others' needs.