Huh, that sounds sticky.
I tend to be more like your husband because (I think) of all the idiotic mistakes I've made that people have allowed me to learn from. Being allowed to learn from one's mistakes can allow you to become SO MUCH MORE then you ever were. On the other hand-so many people don't choose to learn from their mistakes when given that opportunity-and instead they just continue the destructive behavior....
I think due to the sensitivity to the subject for you that it is reasonable for you to not be around M. But in deference to respecting THEIR right to run their relationship on their terms maybe you need to compromise to not saying that they can't be together at the house, but that it needs to be coordinated for you to not be around until they come to a conclusion??
I don't know HOW you would make that work-it's just the thought that popped into my head.