Hmmm....longterm, single-person affair. That's much more serious & painful. I'm torn here. Like you, I'm tempted to agree that a heart can only truly belong to one person at a time -- but I had a friend recently who truly believed he had the capacity to love more than one woman, equally, and longterm. Many people in polyamorous relationships agree with that philosophy. I'm guessing there are truly people out there born with the capacity/propensity to bond with multiple mates. That's unfortunate if one of those mates isn't "into it." The person in your example should probably leave the marriage if the affair won't end -- and it's really bothering them.
