You keep putting the greater blame on the person who had no special obligation to protect your M. This is a terrible way to treat yourself. You deny that your XH had a special obligation to you. You deny that his harm to your M was the deeper betrayal.
I did, for a very long time. I was hoping to salvage my marriage. Maybe that was the ultimate downfall - I thought so little of myself that I forgave him too quickly, tried anything to save it, even in the face of such terrible betrayal. But now I realize that he is a grown man, one who proved himself to have very different morals from my own.
So, I wish they would both pack up and move across the country. However, my children do love their father. They are very attached to him, as he has been a large part of their lives. He is trying to be the best father to them that he can, given the choices that he has made (and, I believe, putting them second to his own selfish wants, pretending that his being happy will trickle down to them being happy. Whatever, he just proved that he wasn't good enough to save their family intact). I don't like either one of them, for the record. But he has rights, and the kids need their father.
This other woman, the girlfriend, has no links or ties to my children. They don't NEED her. She doesn't have any right to be in their lives. And yes, as he ripped apart his own family and hers, she did the same. They want to be together? Fine. That has nothing to do with my children. And if he keeps pushing it on them, I think they will resent having it shoved down their throats as they grow up.
I find it incredible that a formerly-cheating father would want to include their affair partners in their kids' lives, anyway. Then again, I have no idea how they manage to look their own kids in the eyes when they see them, or face themselves in the mirror, either. These people have no shame.
My kids miss their FATHER in-between visits. They could care less if his gf fell off the face of the earth. I think they would just be relieved to have more of his attention, should that happen.
With all that said, in the end, I know that my resentment and anger will fade. Time will do that. I don't share these sentiments with the kids, and I am working very hard at curtailing any pressure that I may have been putting them under about this whole thing. They will come to their own conclusions about their father, the other people involved, and yes, me. All I can do is try to be the best example, and live as I hope they choose to live someday.