My thinking on this is that they've already started the affair by the time BD comes round. They won't admit it but they have been unfaithful and, I'd imagine that inside their heads at least, they've planned an alternate future/fantasy life with the OP.
In Mr J's case he has been involved with OW1 for months before he left (I have long forgot when BD was). He and her were writing to each other, planning their future life together that was going to last forever.
They never lived together and lasted some 15 or 18 months in the open.
For me the problem is not the original alienator, but that some MLCers have more than one alienator/keep having new people.
MLC reality is not always, alienator gone, MLCers crashes and burns, starts to see straing, wants to make ameds/come back. Often the MLCer just moves to a new alienator/person.
Thinking the break of the relationship with the original alienator always equals the end of Replay and the MLCer wanting back may lead a LBS to think that will always happen, when we have many stories on HS that prove otherwise.
Sometimes good things fall apart so better things can fall together. (Marilyn Monroe)