merry - I am so sorry you are going through this! As a mono wife I 100% understand how you are feeling - so know you are not alone! A month or so ago - I gave my husband an ultimatum - her or me. He was heartbroken and didn't want to make that decision - as hard and painful as it is for us - it's hard and painful for them to not have their gf in their lives. Eventhough you are not asking him to break up with her - he still sees her moving out as a loss. It will take time for him to adapt.
I think you have the right to not want to have this woman in your house - I couldn't do it! You deserve a lot of credit for allowing it and for trying to make it work. I wish I had some words of wisdom for you. All I can say is that ALL of you should be in counseling - preferably the same one because that way they'll get everyones point of view and will be better able to help each of you.
Hang in there - if you love him - it will be worth it!
PM me if you need to!