In the previous two posts about the ethics of adultery, we focused on cheating from the perspective of a married person. But several commenters to those posts asked: what about the other person, particularly if he or she is single (and, of course, the other person in the affair is married)?
On the surface, if adultery is wrong, it seems that it would be equally wrong for both parties. But we saw in an earlier post that while adultery is wrong in most all cases, there may be some cases in which it is justified—and furthermore, those cases may not be the same for the married person as for the single.
In particular, many of the moral problems for the married adulterer are based on the commitment itself—the promise of fidelity that married persons make to each other, and the duties to his or her spouse that the married person assumed. But, of course, the "other woman" or "other man" made no such commitment or promise, so this ethical aspect of the problem would not apply in the same way (if at all) to the person outside of the marriage (or committed relationship).
If we're basing our ethical decision primarily on harm and benefit, or consequences in general, then the analysis will be the same regardless of which person we're considering. Consequentualialist evaluations are often called agent-neutral, which means they don't depend on the identity of the person considering the act. If adultery causes a certain amount of total harm versus benefit, and this makes it wrong or not, it wouldn't matter whether we take the perspective of the married or single person. So from a consequentialist point of view, the ethical decision facing the single adluterer is no different from his or her married partner—wrong is wrong, period.
But if we focus instead on duties and obligations—including, but not limited to, those created by the marriage commitment—then we see a different story. These types of moral factors (which usually correspond to deontological ethics) are often called agent-relative, because different persons have different obligations depending on their relationships to other persons (and these may also deviate from the consequentialist recommendation). For instance, a woman's husband has a duty to be faithful to her, while her brother does not; and the brother may have familial obligations that the husband does not have.
What duties or obligations does the single adulterer have in this situation? Just because a person is outside the marriage itself does not imply that he or she necessarily is "in the clear" in that regard. For instance, a single woman contemplating an affair with a married man may feel a obligation towards other women, perhaps out of a feeling of gender solidarity: she doesn't want to make another woman's life any harder. (Of course, a man can feel the same way.) Or, the single person may feel a deep respect for the institution of marriage itself that obliges him or her to respect other persons' vows. But these are clearly less personally focused concerns than one spouse's obligations to the other, and are more contingent on the specific values and concerns of the person involved.
Perhaps the most important personal obligation that the single adulterer has is to herself or himself. Is such a relationship healthy for you? Does it fit with your goals regarding love, sex, marriage, children, etc.? Specifically, is there a chance your co-adulterer will leave his or her marriage? Whether the answer is yes or no, this may have an important effect on your self-image—if yes, then you may feel responsible for breaking up a marriage or home, and if no, you may feel like an occasional companion who will always come second. I'm not saying that the single adulterer should feel any of these things, or that any of these things are necessarily bad, but they should be considered.
If I can leave you with one general thought about ethics, besides my usual point about there being no easy answers and that it all comes down to judgment (which definitely holds here), it's that morality (according to almost any school) doesn't just cover actions towards other people. It also covers self-regarding actions—you should do right by other people, of course, but also make sure to do right by yourself.
So if you're a single person considering such a relationship, ask yourself: "Does having an affair with a married person deny me the respect and concern I owe myself?" If you have any doubts about this, maybe you should cool it down a bit and think about it, because no one can lessen your self-respect except you.