Both my parents had affairs when I was a teenager. My father was emotionally and verbally abusive to my mother. I think her affair was what finally got her out so, far from being horrified by it, I have always been really happy that someone gave her the confidence and self worth she needed to ditch him.
My father, I think, had a string of short lived affairs but the one I know about started when my mum's career took off in her early 40s. It was with a woman down the road younger than my brothers. It repulses me that he found it so easy to replace my mother and didn't even look further than the street. I say 'replace' because that's what it was. When he began to loose his control over my mum and her independance grew, he simply found a new, more vulnerable victim.
I haven't had anything to do with him for years but they are still together. In our old house which, I think, must be frightful for her. I have an awful lot of pity for the poor woman, to be honest, as there is no way he will have changed at all. I even felt sorry for her at the time, knowing she was being sucked into a loveless relationship where she would never be anything more than an emotional punch bag for a hard-hitting bully.
It's not fair to say that children's reactions are invariably horror at the parent's actions. Few things are that cut and dried.
I actually feel terribly sorry for the OP. This must be a truly horrible way to live to the point where I wonder whether there's a degree of masochism involved. I am sure that, whatever the outcome, her children would love her to be a much happier person unburdened by all this pain.