Do you believe in cheating?

[Angrboda]

You're very vague with your terms. Cheating in an open relationship is an oxymoron, so I suspect you didn't ask your question properly, or perhaps, you intended the ambiguity so that you could use it to advantage in the discussion.

I think that when you say cheating in a relationship, most people automatically think of a committed, monogamous relationship, as that is the most common kind of sexual or romantic relationship referred to by the word "relationship" here in the Western world. Like any other moral in human cultures, the ethics likely reflect evolutionary realities, and feelings of right and wrong have developed to reinforce behaviors. In a committed monogamous relationship, especially one such as a marriage, you see a perfect storm of evolved psychological mechanisms, all of which are "designed" to maximize the reproductive potential of the individual's genes. There's reciprocal altruism and tit for tat, including a constellation of social behaviors designed to make cooperation more successful than defection. There is sex itself, which results in hormonal storms in the brains of both partners, giving them incentive toward pairing and social behaviors. There are the hormonal and social effects of having babies (which follows from having sex) which encourages monogamous, stable bonding, so that any offspring have the best chance of survival. And there are the rituals, which cement social roles and are held in place by incentives and sanctions.

So, while it may be true that cheating is ethically wrong, cheating is discouraged at many levels because in the context of a social species which practices K-selection reproductive strategies (few offspring, heavy investment), cheating and behaviors that give rise to or enable cheating directly threaten reproductive fitness, so many mechanisms from cognitive to hormonal ally against such behaviors.