I am also not married but have been with my boyfriend for 7 years. I think I could live with his mother. She would not be nosey and would respect our privacy.
Now, he probably (and I don't blame him) wouldn't want to live with my parents. But I don't think we need to worry about that, so we are safe.

That's great that you get along with your MIL so well. It really helps your relationship with your husband. I know so many people who can't stand their MIL and they make them miserable (and they don't live with them!).