what kind of person would pursue a married man/woman? How could you ever trust that kind of person in a R? Early on I wanted to ask OW how would you feel if some single woman chased after your father and tried to steal him away from your mom???
Nobody gets "stolen" nowadays, so we need to consider that our WASs were active, willing participants, rather than consider the OPs. Both act selfishly. There was a point in time when my ex's EA was discovered, after which she allegedly did earnestly want to "work" on the marriage, and cut off the OM (I was so informed by a third party much later on). OM's reaction, I was told, was that he went "ballistic" and tried to contact her any which way he could. Who's interests was the OM pursuing at that time? Truly not my ex's stated interest.
So, yes, they're together now, but again, what do you have? Some guy from her past who's been meeting her emotional needs, but yet, is someone who cheated himself on his relationship and encouraged her to cheat, and she being someone who showed him that she's capable of the same. What happens when in the normal course of a relationship things get difficult? People tend to do what's easiest for them, which is to replay ingrained behaviors, and since their innate behaviors tend toward selfishness, the old adage "If he/she cheated with you, he/she can cheat on you" comes to mind, and will probably come to both their minds too. Certainly, such selfish traits probably play out in other ways as well between them.
So, by way of that, this adage comes to mind: "The best revenge when losing a mate to another, is to let the OP have them." It comes down to just how much they can take before, once again, the bad outweighs the good.