A small minority of people claim that this is possible but we've never seen it to be true over the long-term. In fact, human nature is such that this cannot work long-term. Long-Term Love Relationships are defined by the intimacy and commitment that two people share with each other. Showing that intimacy and commitment with others lessens the special bond they have with each other, to say nothing of the feelings of jealousy, fear, sadness and anger that affairs create even when they've been "agreed to."
The author of Intimacy after Infidelity: How to Rebuild and Affair-Proof Your Marriage (2007, New Harbinger Publications), Dr. Solomon received his undergraduate degree at Dartmouth College and his M.A. and Ph.D. in Clinical Psychology at the California School of Professional Psychology (Los Angeles). He has been in private practice in San Diego and La Jolla, California for more than 20 years.