Well, it seems this thread has run it's course.
So I'll just give my take on the original question.
The idea behind "affairing down" is that APs (affair partners) are low-hanging fruit. Not a lot of people are willing to be an AP, so the cheater has to take what they can get.
Whether or not there is merit to the idea is another question.
If you really want some sort of theory behind it, I'd say it has something to do with the AP's lack of self-respect, confidence and self-worth. If they're willing to tolerate the attached party trot out a myriad of excuses, as to why leaving /divorcing is not possible at the moment, if they're willing to settle for breadcrumbs instead of a whole loaf for an extended period of time - that says something about their sense of self-worth and respect.
It has nothing to do with looks/intelligence/class/money.
Most people would not admire or respect a cheater and a married /attached person knows this. They realizes their relationship status is a huge negative when it comes to attracting an AP. If they want to have their needs met in an affair, they must find someone that will admire them, respect them, love them, accept them, compliment them and be attracted to them regardless of the fact that they're married and cheating.
It's all about them and what they want to feel like. A disappointed partner/spouse makes them feel like a failure, but an AP who doesn't expect much makes them feel like a king/queen.
Of course isn't true in all affairs, but most cheaters are not great people stuck in bad circumstances. Many are just flawed, selfish individuals who make a mess of their lives and drag down and hurt those that love them.
The big issue is the power of deceit. The cheater uses deceit to keep the partner/spouse in the dark and the AP buys into the deceit and needs to believe that this deceit is not a true reflection of their character.
Cheaters seek to meet their needs, by looking externally for what is an internal problem. It doesn't follow that those needs are valid or realistic.
It's not easy to provide an endless supply of respect, compliments, sex and attention to someone who may have behaved in ways that you no longer admire. If someone has a pattern of broken promises, irresponsibility, immaturity. etc
The spouse/partner might not be as willing to supply as the AP, who hasn't been disappointed yet. This is why when the AP becomes disillusioned, when they have expectations and start calling the cheater out on the failure to deliver, the cheater will often seek another source of supply.
Many cheaters may choose to cheat based on their perception of unmet needs, overweight wife, husband is lazy, not enough sex, etc. - but these are just excuses.
The real reason they makes this choice over other options like marriage counselling, divorce, an open marriage, individual counselling, is because of their character and lack of integrity,
A sense of entitlement, unrealistic expectations of marriage/partnerships, immaturity, passive behaviour, unhealthy needs for attention and validation, conflict avoidance, poor coping skills, being emotionally unavailable and empathetically bankrupt are explanations that can help understand the choices cheaters make,