I've forgotten where I read this, but it was something along the lines of - with our spouses relationships with OW/OM being built on lies and deceit, this puts it on a very different platform than the relationships that we built with them over the years.
From the word go, these people will be waiting and watching our spouses for any signs of the same behaviour (well, not too far into their R anyway), and this is not a position from which strong roots and foundations are built.
After being together for so long and having such history, we're dealing with something completely different than the superficial replay R with OM/OW.
My H has said on a few occasions how selfish both he and OW are - nice!!! and that when he left he was no longer capable of having a R with me and - in fact I'm not capable of having a R with anyone ....... 'Well no sh*t sherlock' as Affaircare said somewhere .......