OP
I'm not going to flame you, but I am going to tell you something of my experience, so you have an idea of some of the things men in this position do/say to all involved.
My exH had an affair. He is now with the OW. Not because he 'left me for her', but because I discovered it and kicked him out as soon as I knew; there was no "what shall I do..?" type consideration.
He was 'devastated'. He told me he never had any intention of leaving me for her (he was very contrite); he moved in with his parents, not her; he apologised to me; he turned up on my door step unannounced telling me he still loved me and begging me to try again - I asked if she knew he was with me telling me this. She didn't; he ended it with her; she had 6 months off work with stress; they both lost the respect of their families (my exH's father barely speaks to him even though they live in the same house); her family "hates" him, his family "hates" her...
When it became clear to my exH that I would not even consider taking him back, he resumed his relationship with the OW. His reasoning being that he had "to try and salvage something" from the situation.
FFW 18 months...
He still lives with his parents. All is not well in paradise. When I said I thought that him moving in with her was the plan, he said, "No. She's my girlfriend, but that's all. It's not really like that." I wonder if she knows that..? He is with her to "make the best out of a bad situation"
I am still friends with some mutual friends. I know they have met her and like her and are nice to her, but, in their eyes, she is the interloper and they don't 'respect' her.
IIRC, 'your' MM isn't even making promises to leave his wife.
Surely, you deserve better than this! His wife and kids deserve better than this. You, however, have the upper hand in that you know for definite he is a lying, cheating, manipulative, thoughtless piece of shit and his wife doesn't. If he loved you, and not her, he would leave her. You are meeting a need that his current situation isn't, but that is all.
For you, this is your relationship. He already has a relationship. You are his hobby; his escapism. Please, do yourself a favour and find someone worthy of your love and commitment. Take care.