In either case, people don't feel free to admit being attracted to someone else. If they don't admit these attractions, then they won't admit being tempted. If they don't admit being tempted, then they certainly won't admit it if and when they finally act on the attraction. The effect on the relationship is to cause it to be filled with jealousy and suspicion, as well as making it less likely that it will be monogamous.
Bottom Line: There are no guarantees. The issue of monogamy is never settled once and for all. It requires ongoing honest discussion of the issue. This makes it possible for a couple to feel they really know each other, making it more likely they can trust that they won't deceive each other, thus preventing affairs.
All the above adapted from "The Monogamy Myth", Copyright © 1989 Peggy Vaughan
Printed here with permission
Her web site Dear Peggy has numerous articles on the subject of dealing with affairs.