brokenhearted - RCR's discusses the "affair down" in one of her articles. Affair downs sometimes pre-exist or are created by the affair dynamic itself.
So, a person could be an affair down in terms of looks, education or social standing. Or they might not be. But someone who is willing to get involved with a married person and conduct a "secret" relationship behind the backs of a spouse and often children is engaging in "affair down" behaviour (so morally speaking it is an "affair down").
Not only that, but they have accepted a person that they KNOW is capable of deceiving the people that they love into their lives, sometimes for a long time. So, they often will gradually engage in "affair-down" behaviour (jealousy, paranoia, controlling behaviours) that they may not have engaged in before. Anyone who thinks this is the dynamic of a healthy relationship is already damaged in some way - another thing that makes this an "affair down".
I have no idea if my H's OW is a nice person (I assume that in some areas of her life there are people who believe that she is). She is younger than me and him, she is reasonably attractive and reasonably well educated. But I know that she has big abandonment issues that have damaged her - H gets this ridiculously OVER sympathetic glaze across his eyes when talks about her "problems", as if they justify her actions. I don't know what she was like in her previous relationships (I actually don't know that much about her), but I have witnessed "affair down" behaviour in her. Watching H like a hawk, determining how long and when he can spend time in my presence and on the phone to me. I am sure that he has had to bend over backwards to accomodate her paranoia (but being in MLC he currently confuses controlling and clinging behaviour as "love".)
I don't know your situation, but my guess is that you would never contemplate dating a married person, broken. So, at least in the sense that she either does not have, or does not live by, good moral values, she is an affair down from you.