I could certainly paint a scenario in which that sort of behavior would be reasonable; to wit: someone is married to someone who is disabled in such a way as to not be able to be intimate, to satisfy their partner. An open affair would devastate the disabled partner, who wants and needs the care the cheating partner is more than willing to provide. In this case, it's something like "don't ask; don't tell." And once you realize that such corner cases exist, you can see how the line could be drawn arbitrarily. The evidence is, a lot of this stuff happens. You could almost call it a human trait. We're a complex lot, not easily fitting into slots others design for us.