Monkeybot

The wrongness of cheating isn't the sex, it's the lying involved. Lying is considered by most moral systems not to be a favorable thing to do.

If A promises to be faithful to B (explicitly or implicitly), A should live up to those promises. If A does not, B may experience hurt feelings and anger. B may also find it difficult to trust people in the future, and this may hamper B from entering into future healthy relationships. B may have put significant investments in the relationship, such as money, time, and emotions; these might all be considered nil after A proves himself to be an untrustworthy person. Moreover, if A isn't too discriminating in his choice of people to cheat with, A may inadvertantly bring home a sexually transmitted disease to B.

If A is upfront with B and says s/he wants a sexually open relationship, s/he is being honest. B can make an informed decision whether or not to accept A's choice, and can prepare accordingly (i.e. always using condoms or what have you). There is much less emotional trama here, less risk of disease, and a more honest relationship between A and B. Why wouldn't this be the preferable situation?

openeyes

I think Monkeybot and brighid make excellent points.

It's the dishonesty that's the big thing. If you enter into a relationship with person A and that person has certain expectations and changes his/her life based on those expectations, then it's wrong for you not fulfill those expectations without some discussion. If you are purposely not being upfront with your sense of the relationship just to secure the cooperation of person A, then your character is questionable.

Oxymoron

I think that it's even more than just the lying as a principle. It's the expense of energy weaving the web of lies that might otherwise be spent on being nice to your partner or on neutral activities. Lies have a habit of multiplying. Having come back from an illicit tryst , one must lie about whereabouts, what one was doing and a whole bunch of circumstancial stuff that makes one's deceit look convincing. Being unfaithful really does require dedication, and that stings as well as the deed itself.