the "one secret extramarital affair is more honest than open swinging" defenders — your actual counterarguments please

so my brother in law (sweetheart of a man, married to my sister 9 yrs, fully monog as far as we know) found out we're in the LS last christmas (sister told him, sister was fine, BIL had Feelings). and the thing he keeps coming back to in conversations since is this argument that ive heard variations of from religious-ish friends and even some therapists ive read online. its something like: "a single discreet emotional affair is more ethical than open swinging bc the affair is contained, doesn't spread risk to multiple people, doesnt make ENM a normalised lifestyle that affects how my own kids see marriage, and is usually a response to a real wound in the marriage rather than a recreational choice. swinging is selfish bc its sex without need." and i wanna be honest. i didnt have a clean comeback. i had reactive comebacks ("that's gross because affairs involve lying") but i didnt have a thought-through argument for why open ENM is structurally MORE ethical than a contained affair, not less. when he said "you're still lying, just to your kids and your neighbours instead of your spouse" i kinda stalled out. so. for people who have actually thought about this — what are your real counterarguments. not "affairs are wrong" (i know affairs are wrong). but "open ENM is more ethical than affairs ARE, and here is why". im trying to be able to defend this position next time it comes up at family dinner without just getting flustered.