The thing about No Contact is that it must be BOTH ways.
HE needs to know what you are doing and WHY. He won't accept it, he will fight it tooth and nail, but he needs to know the score before you cut him out... and EVERY TIME he tries to VIOLATE your wishes by trying to contact you - AND HE WILL - You repeat that position in email or a note of some sort and then dissappear.
I would visit MIL privately when he is NOT there, offer her a gift wish her your best, and vanish.
Establish a third party location where your H can drop things off to you, and you to him... Where you won't see each other... for example at MIL's.
This is how that would work. HOWEVER. Given that he IS turning to you.. this is an interesting one...
I don't believe him when he says he doens't want to be married. What he MEANS is all he can see is the negative aspects of being married while carrying on an affair and he doen'st feel good... duh.
Marriage DOES have a positive light if you look for it and enjoy it... THAT brings back the good feelings again.
Right now he's wallowing in his misery like a pig in the mud.
The thing is... your H has kids so he IS married whether he likes it or not... I suspect the responsability for his children is weighing very heavily on him right now and he hates that feeling... He needs to grow up. Responsability is an unpleasant feeling when you have the maturity of a 12 year old.