Is it fidelity or morally wrong if both partners agree to swinging?

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I would first like to make my view clear, that I do NOT condone any type of relations outside of a union/marriage. But this question has made me curious what others may think about that. I recently cut off all ties to a married couple who did that and weren't afraid to talk about it...so much that everyone in the neighborhood knew and people I didn't think knew them, knew. The phrase if it walks like a duck and quacks like a duck....I don't want to be associated with that kind of thing.