What is wrong with cheating is the lies to the other part in the relationship, not that one chooses another partner - It's a free world. It's perfectly OK to be with another man/woman if one is married or in a relationship, as long as the other part in it is told of the "infedelity" at first chance. Then he/she can openly decide how to deal with the fact, and wheather to stay in or leave the relationship/marriage as he/she sees fit.
We are all free men/women, our partners do not "own" us like slaves, and so long as everything is laid bare, I can see no problem in breaking off a relationship by jumping another man/woman (or staying in it with an unfaithful partner if that is ones choice). I would not blame a girlfriend who found another man more attractive, as long as she immidiately told me, at which point. This would make her life better, and allow me to choose to "better" myself - or leave and find a woman who my current self would truly satisfy.