Every guy that has this asked of him, instead of acting mad and hurt, should act totally excited and say "yes I have been looking to fuck some side honeys, thank you for bringing it up" That would stop these dummies in their tracks.
That doesn't nip it. Nipping it is "I'm already in an open relationship but I demand that you not get fucked by other men, don't like it - leave". Men and women aren't the same and don't give up the same things to be in a relationship - a faithless woman isn't a wife / gf.