You ask a good question and you are right that only your H really has the answer. I am not sure that I can hazard a guess. Many people on this site have experienced the almost automatic turning off of emotions and love by a WAS. There are usually some signs that something is amiss but they do seem to transform rather quickly from someone who is sleeping next to you and interacting normally to someone with a cold, vacant stare who is hanging on to the edge of the bed in a desperate bid to put as much distance between you as possible.
As for the "shamming" etc, a lot of WAS do rewrite history. Maybe it is an attempt to ease their conscience. They probably did have some aspects of the marriage they were unhappy with but it was their responsibilty to voice them and seek resolution. I don't buy the "shamming" argument for one second.
I know that you will want answers to all the whys and how comes but don't drive yourself crazy trying to figure it out.
Don't let the loss of "desire" on his part make you think that you are anything less than a desirable woman. This is his choice now. Remember that even the most beautiful and desirable people in the world have had cheating partners. It did not make the LBS any less beautiful or desirable. Reassure yourself that you have inner and outer beauty. Continue to grow and work on yourself and realize that it is his loss if he chooses to walk away. This is where you focus on yourself and your children.
Keep reading. Keep posting if it helps. If you can post on other people's threads you will get more traffic and comments on yours.
