The thing is that MN is very keen on generalisations. They are lazy and treat all of humanity as a single entity. With single reasons for behaviour without consideration to the fact that as individuals there are also individual circumstances.
The oft trotted out lines of ;
Marrying the mistress creates a vacancy.
Once a cheat always a cheat .
Are no more accurate than ;
Leave a relationship first before starting another makes a better relationship between the children and ex spouses.
I know very many relationships where that has happened and STILL there is bitterness and anger from ex wives and children. Because No one likes rejection, no matter the circumstances that lead to it.
Peoples circumstances are also complicated and affairs happen for all kinds of reasons. It is not always a man or woman looking for a sexual thrill or to get away from a dull or boring marriage because family life can be 'too much effort'. Although that IS the case for some. Again - individuals have their own moral code. So yes , someone who enters a relationship and bails to a new one the moment a bump in the road is hit , will probably follow the same pattern again and again. but not all.
In my village alone there is a man 'having an affair' living with the 'OW'. His wife has been in a care home with a degenerative illness for over 25 yrs. He visits everyday. She has little knowledge of who he is now.. he loves her and will never divorce her . Is he wrong to find happiness? Is his partner 'the OW'. ?
My friends brother is also having an affair. He is in a very unhappy marriage with a woman with serious diagnosed OCD who will not even think about therapy. Her disorder affects him and their children. She is from Eastern Europe. If he leaves she will take the children to Estonia. He's chances of preventing this through the courts is minimal due to the lack of family support here, and the 'damage' psychologically that a court directive forbidding the removal of the children would cause. He has taken advice and been told that this is almost certainly the case. So stay married or lose your children. Is an affair that difficult to understand in those circumstances?
Would either of those people 'replace the vacancy ' or 'always be a cheat'?
I doubt it very very much.