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LW, if you truly love your wife, and if you think she truly loves you but has a weakness for this guy John, do this: get John alone somewhere. Tell him to stay away from your wife. Tell him that you'll rip his balls off and shove them down his throat if he doesn't stay away from her. Scare him.
Do this where nobody else can hear you, so you can deny everything.
Violence is a crude tool, but effective. Threats of violence can be just as effective.
John is in the wrong, absolutely and completely. Don't feel bad about scaring the piss out of him. If you have a good friend, preferably big and scary, bring him along. Do this, and John will go away and pester someone else's wife.
Good luck, dude.