When you said, "enjoy it while you can." that is exactly how I feel. I am going to enjoy all of this while I can. MM knows of the type of relationship I want, but I think that scares him a bit. And frankly, since he has the type of marriage just like you described, where they are the perfect couple when out but at home they are like complete strangers, he deserves to have someone who can love him and devote to him all the time. In my opinion, if MM were to leave his wife for us, he'd just be trading one miserable lifestyle for another because I wouldn't be able to fully devote all of my attention and love on him. He'd have to be willing to accept what I can give, and in an essence that is what he is doing with his wife. That's not good enough for him - my opinion again.
So while I would love for this to never end, it probably will and not in a pleasant way. However it ends, I will be better for having had a relationship with MM rather than always wondering what it would have been like.
Anyway, my 2 men make me so happy and for that I am grateful.