He is an alcoholic doing what alcoholics do. Trying to make sense of it is futile. My father left me and my mom and moved in with a drinking friend and her 4 children. I was about 9 years old at the time. It makes no sense and yes, it is totally unfair to the family left behind, but it is what it is.
It's good that you are in counseling. Take care of yourself and your sons. That is all that is important now.