Having an affair/cheating on an S/O or spouse and hiding it/lying about it is a violation of trust... to me that is simple.

Those who view it as being o.k. don't have a factor of trust violation in their understandings between each other... I don't think of it as a jealousy factor... just an understanding between 2 people agreeing that the particular behavior...."affairs/cheating" is o.k.
Although they have different terminology for it.....

I don't judge what other's behaviors are.... it is their choice.... Would I do that.. no. There are many mitigating factors involved individually for a person to decide "cheating/having an affair" is o.k.

If I were in the frame of mind that monogamy wasn't for me.. I certainly wouldn't be with a person whose beliefs were totally opposite....

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A blessed thing it is for any man or woman to have a friend, one human soul whom we can trust utterly, who knows the best and worst of us, and who loves us in spite of all our faults.