Although we say that OW is nothing but a bandaid... a distraction... means nothing.... would be someone else if not her..... I believe that many times the MLCer is working through some FOO issues with his particular OW/OM.... In the beginning they are attracted to qualities that seem to be admirable to them, but as soon as some time has passed, they often times complain about OW/OM as being just like their MOM, or too much like THEM....
This is only a theory based on a very small amount of anecdotal evidence.
Given that the OW/OM DOES actually serve a purpose, stopping it from being a PA doesn't really make sense.... EA or PA, it's still a betrayal and separation from the marriage... bringing a third person into the mix... giving them importance.... allowing them control and influence....
For the record, I absolutely HATE the PA..... I have resisted accepting this part of the MLC process tooth and nail.... I HATE it... but after 19 months, I'm seeing that the power has shifted BACK to my marriage and to me, so I'm not as threatened by OW relationship.... I know where she's headed. I still hate it, but I know he is working some stuff out with her (yuck!).... so now, the PA is essentially just a used tissue that needs to go to the rubbish bin.... eventually.
Don't focus on trying to stop it.... you really can't.