My thoughts? No! If both parties have full knowledge that they are, in fact, having an affair, the single person is just as culpable as the other. Some may say that the person committing the adultery is more guilty of wrongdoing, but c'mon, we're talking about consenting, privy adults here who know what they are doing! Consider this, if you are single and having an affair with someone, you are fully aware that what you are doing is wrong because many things about the relationship must be kept secret. And people who have nothing to hide, hide nothing.
As if that isn't enough, consider that, while you are involved with someone taken, others are working on blind faith in you. Perhaps your children, work colleagues, family and clergy members. Or worse, possibly even the spouse of your lover. And I can't think of anything more duplicitous than that!