I have often wondered, in my situation, being with someone who (when we are together) cherishes me and treats me with great esteem, affection, and attachment, what precisely hurts when that person must go and be dutiful to the rest of his life (read: the partner to whom he is committed).
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.
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 they leave, it's their OWN DECISION to step out of the marriage. Accept that and you will understand that you really weren't married in the way you perhaps thought you were. White, thanks for continuing this interesting thread.
By definition alone a single person can't commit adultery. This one reminded me of other articles on the PT site that discuss the issue of bullying, specifically how the bully is the one most hurt by the bullying.
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 , 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?
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?