Swinging isn't NTR. The closest equivalent (That I can think of at the moment) is a free-use, free-love hippie commune. You share your partner with someone else, and they do the same. But it isn't NTR until those partners decide they like the other person more (Until Couple 1's wife/GF/husband/BF decides they like Couple 2's equivalents (And vice versa) more than their own partners).
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