Kialo requires cookies to work correctly.
Is Adultery Ethical?
Yes; even if you're not responsible for the married person's choices, helping someone to do something wrong is wrong.
If you know someone is married, you shouldn't sleep with them, because there is a high probability their marriage will be ruined as a result, even if it's possible it won't be.
You do not know if there is an understanding, if they are separated, or even, necessarily, if they are married. Unless you are informed that it is wrong you cannot determine it is wrong.
The moral choice of what is wrong or right is not objective.
This assumes that the married person is necessarily doing something wrong by sleeping with someone other than their spouse, which isn't necessarily true and certainly isn't always the case.