I believe that the reason an affair relationship can last has a lot to do with whether there is a standing spouse, whether the MLCer feels he has done too much damage to return.... a lack of trust and forgiveness.... not to make it all the LBS's fault, because it's not...
My own husband has confessed that if I won't have him back he will be "stuck with OW".... now he is saying he will NEVER stay with OW for life, but if I won't have him, he will have to find another woman right away.... he cannot be alone.
My friend's Dad left her Mom and married his affair partner (young secretary) 25 years ago. The alienator immediately got pregnant in order to either legitimize the realtionship and stake out her territory, or simply because young childless women might want to have children... something the older MLCer never things of as a consequence, hahahaha!! They leave their families to be waited on and adored by a young woman, only to become displaced by a new baby... but I digress... anyway, her Mom has also had a boyfriend for about 15 years and says "no way" to the idea of her ex husband.... however, my friend says her Dad, though content, has expressed MANY TIMES that he would prefer getting back together with her Mother... In their case, he is content enough, and unwilling to upset the apple cart and start all over again...
So, of course the affair relationship can last.... I believe it is the MLC affair that rarely lasts, because it is the perfect storm coming together of two F'd up people!! If one gets well, or wants to get well... that ends it. My husband has woken up to discover his OW doesn't have any redeeming qualities, other than she works a lot and "I guess that's a good quality... maybe I should think about it some more... surely there is SOMETHING about her I like...."
and he said this to me with a straight face!
The reason the MLC affair lasts so long is because it is an emotionally bonded affair.... in a "normal" affair, it is all about the fun of secrecy, and once it is exposed to the light of day, the two parties generally decide IMMEDIATELY they didn't want to lose their marriage, home, family, job, reputation... they only wanted a little fun on the side for awhile... exposing the affair generally busts it up right away... not so with an MLCer.... they are in a romantic, fairy tale fog..... I think Mamma Bear's BIL described the affair as "The first 6 months it's all fairy dust and lalalala... then after that.... it's no fun, but you can't get out..."
I also imagine the reason so many marriages IN GENERAL don't last is the propensity to encourage people to chase "personal happiness" at any cost...