I'm not going to discuss how they are in real life. But for most people, even those who allow for open relationships, misleading a lover/spouse/significant other is, frankly, bad form.
So the adulterous lover, Joel, in Law & Order, Season 2's "Blood is Thicker" starts with strikes against him.
His lover, Lois, the victim, was killed by her dilettante, upperclass husband at his mother's instigation. At the end of the episode, Joel identifies the clasp or family heirloom that the husband took from his dead wife's body, proving that the husband was the killer.
What makes Joel infinitely touching is not just that he accepted that his lover, Lois, was using him to make her husband jealous--and not just that he is played by Joel Polis--and not just that he finally steps up and tells the truth--
What makes him infinitely touching is that he seems to be the only person who actually misses the dead woman. Even Stone and the police are after the murderer because they despise him, not as payback on behalf of the wife.
In comparison, Dr. Joel Friedman actually cared for Lois. He was her confidant (in fact, he was likely her confidant before he became her lover). He knows her secrets: things she cared about, things she hoped for regarding her family, her future, and her children. To him, she was beloved.
The final scene in court is one of the best from original Law & Order--due to Joel Polis as a fundamentally decent man, for all his flaws.
