Insanity is not selective. People aren't "insane" in one area of their lives and sane in every other. There are more possibilities, but we have to be willing to look at them and accept them, particularly as time goes by and it's clear the MLCer does not have a serious physical or mental disorder.
Many LBSs start out thinking their spouse must have a brain tumor or be completely losing their mind. The alternative is too painful. But, over a long enough period, that reveals itself not to be true in the majority of cases so sticking with that narrative is not healthy for the LBS's own progress and healing, in the absence of a genuine scientific diagnosis. Many, if not most of us, have asked, "What if my spouse is insane?" At some point, we have to also be able to ask, "What if they are not?"
My father did the same thing and then 35 years later told me that he just wasn't content in his life, he loved my mother, but did not love being married and tied down. He loved his daughters, but did not love being a father. He wanted free of responsibilities except for those he chose for himself and he didn't want to deal with all the upset my mom and sisters and I were going through. He also admitted he and the OW were already unhappy less than two year into their relationship, but they had convinced themselves they were soulmates to such an extent that it was worth blowing up multiple people's lives and they didn't want others to know that they were the cliche everyone told them they would end up being. So for pride or definance or embarassment..they chose to stay together 15 more years. Perhaps because it finally became clear that the grass is not greener and each relationship becomes routine and filled with responsibility over time no matter how new and exciting it all seems in the beginning.
My father eventually (decades later) told my mother and my sisters and me that he regretted what he did and always would, and that he knew it relatively early on, but did not want to do the work to fix it or live with the constant reminders of what he did. It was better for him to set that part of his life off to the side (compartmentalize) and move on and leave us to a life he convinced himself would be better for us.
He had the hallmarks of MLC, but wasn't insane. He was broken in childhood, refused to do the work to address it, carried issues into adulthood where he self-medicated with coping mechanisms such as women/sex and other escapism. He wasn't strong. He wasn't selfless. He wasn't faithful or committed. He develope depression he did not seek help for, but he wasn't insane. Would it have been simpler or easier to believe that he was? Maybe in the beginning, but not over time. I eventually reconciled with my father and he was a very different man with my daughter and a wonderful, loving dedicated grandfather. He finally grew up, got help and became the man that my mother and his daughters deserved, but far too late to save our family or leave us unscarred.
I agree that sometimes diagnosable physical and mental illnesses are initially thought to be something like MLC when they are not, but time reveals. in the majority of cases on this site, that it is not a degenerative mental and physical health issue, but more a crisis of another type, including depression.
Your spouse may have a degenerative mental condition and that will become diagnosable over time if symptoms and brain atrophy progress. In my situation, I know that is not the case because of the amount of time that has passed and the absence of all other symptoms.
Self preservation is one of the strongest motivators humans have. Not only preservation of our physical lives, but the preservation of our view of ourselves. For many MLCers, they are uable to face their actions. Knowing my spouse for 31 years, I am not at all surprised that in the face of all that he has now done, he would rewrite history to justify his behavior and comfort himself with the erroneous narrative that it is D who has abandoned him, rather than the other way around. He is unable to reconcile his need to be seen as a good father and a good man at the same time his actions contradict both. He was never particularly strong or resilient nor even perhaps connected to people in a deep enought way that would tether him there tight enough to withstand a storm such as MLC.
He is not, however, insane. He has his reasons and while they may not be reasons that I would succumb to or find acceptable, he is not me.
I don't debate that your story is different from mine. As much as there are universal themes, every person, every family and every set of circumstances are unique.
I wish you peace, ChrissYAH, and definitive answers for your journey.
Phoenix