If love of others is taken to be a supreme value is there any ethical justification for a mature (50+) married man and woman to love each other when they are not married to each other (adultery) assuming children are not at risk (grown and gone or don't exist) and that great care is taken to keep the relationship secret and private (assume both live in distant countries/cultures)? Assume neither pregnancy nor disease are issues. Assume both people live in passionless but stable and friendly situations and recognize legitimate needs of all concerned to social and financial stability. Both divorcing would likely cause much pain and disruption to many. Terminating the relationship would deny both their last chance of expressing their mutual affection, of sharing their highest value. From at least Aristotle on, and for most religions and cultures, adultery is a no brainer - wrong without question. With aging populations in a modern context this question will become more common. If it is wrong, how serious is it?

I'm not sure why one would suppose that "love of others"---by which you seem, obviously, to mean romantic love---is a "supreme value", expressing which over-rides so much else. That said, yes, this kind of question is very real. And surely it's possible for someone to decide, in certain circumstances, that it is better to have an affair than to destroy a marriage, neither option being a very good one.

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