I assume most people will agree that it's wrong to cheat on your partner. But is the person you cheat with doing anything wrong? Indeed, is it really the act that matters here?
Now, I don't think there's anything necessarily wrong with non-exclusivity per se. Rather, I take "cheating" to essentially involve dishonesty. What's wrong is to mislead your partner into believing that the relationship is an exclusive one, only to violate that expectation behind her back. Such deceit implies a gross lack of respect.
So consider a would-be cheater who simply lacks the opportunity to act out their adulterous wishes. This person is no better, it seems to me, merely for being constrained by circumstance. It is the intention to cheat - should the opportunity arise - that constitutes disrespect. The act itself seems superfluous. (I think Jesus once said something similar about "committing adultery in your heart", right? Any Christians out there are welcome to comment with further details.)
If this is correct, then it seems that the wrongdoing has already been done long before the actual act of adultery takes place. Hence one's partner in infidelity is not really a participant in wrongdoing after all. Does that sound right?
One might object that the act has significance for the cheated partner. They would be more devastated upon learning that adultery actually took place. This is arguably irrational, at least if the above line of argument is correct; they should be just as devastated by the mere intention of their partner to betray them. But rational or not, the felt pain is real enough, and this may be a morally relevant factor.
Are we obligated to avoid causing people irrational mental anguish? I'm not so sure about that, actually. Appeasing irrationality may be a bad policy, as in being overly sensitive to "offending" homophobes or religious sensibilities. In general, I think it is important to uphold that individuals are not responsible for others' irrationality, and so not (wholly) responsible for their irrational suffering. But this may be a special case, for even if the cheated partner only has legitimate complaint against the intention, and not the act, still the two are closely enough related that causing pain by contributing to the latter might also be morally problematic. If they are already a victim, it may be callous (even if strictly true) to dismiss related hurts as "irrational", or to be careless about exacerbating their situation.
What do you think?