If you decide you need him to leave, you can do that while making it clear that you want the M to work. It comes down to self-respect. As in, "I love you and I want this M to work, but your involvement with someone else is not acceptable. As long as that's your choice, I need you to leave."
In some ways it's harder on you with them in the home(IMO). Once they leave, well, that may be it. You stil have to respect yourself. My H left on his own and as far as I know there was no EA/PA while he was there. I will tell you it was easier in some ways after he was gone (except the day he comes to visit, long story) and I felt like I could breathe a little easier.
Whatever you choose think (and I mean really think) don't react.
