So your wife would supposed to be your best friend.Not a chance. I’d never betray a friend like that. Not worth it
Very excellent point. If someone does not have a conscience, how can you convince them much of anything is wrong? Perhaps it's a useless scenario then. Just as you cannot logically reason with someone who does not value reason, arguing with them would be a waste of time. That said, I think Sam Harris did a really good job at the end of his short book "Lying." (emphasis mine)This is why people who ask things like "what's the problem if no one knows?" can't be given a good answer, because you either have a conscience or you don't. And if they had one, they wouldn't ask that question in the first place. The feelings of guilt and remorse would in of themselves be the problem letting them know it's wrong. But their brains don't work like that. If you don't feel that betraying people behind their backs is wrong, then there's nothing anyone can say to "logically" convince you otherwise.
However I do make a distinction between you breaking your vows, and someone you sleep with breaking their vows. It's the responsibility of the committed person to not cheat, not the random single person.