Scenario: suppose a man/woman gets caught cheating on their significant other, and the third party involved knew that the man/woman was in a relationship. Who's at fault?

The man/woman for stepping outside of their relationship?

The third party for getting involved despite the fact that the person is in a relationship?

Or both?

I only ask because I have seen time and time again on this site where women claim that the person who's cheating is at fault, and that the person they were cheating with isn't, because they aren't the one in a relationship. I can understand that viewpoint if they weren't aware that said person was in a relationship.