Married. How many guys have a side dish ?

[Calihopeful]

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.

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)

"Lying is, almost by definition, a refusal to cooperate with others. It condenses a lack of trust and trustworthiness into a single act. It is both a failure of understanding and an unwillingness to be understood. To lie is to recoil from relationship.

By lying, we deny others a view of the world as it is. Our dishonesty not only influences the choices they make, it often determines the choices they can make—and in ways we cannot always predict. Every lie is a direct assault upon the autonomy of those we lie to.

And by lying to one person, we potentially spread falsehoods to many others—even to whole societies. We also force upon ourselves subsequent choices—to maintain the deception or not—that can complicate our lives. In this way, every lie haunts our future. There is no telling when or how it might collide with reality, requiring further maintenance. The truth never needs to be tended in this way. It can simply be reiterated."