I'm not terribly worried about moving in affecting chemistry and everything. It is a factor, to be sure, but I generally keep to myself and they know to leave me alone when I need it, so the only thing in this area that I'm worried about is the baby killing my sleep.
My girlfriend already has a husband, whom she lives with. We're good friends, practically brothers. She's not looking for another relationship, though if it did happen, I wouldn't mind as long as he was respectful of my boundaries (as in, allowing me to sleep) and I still had my own room.
I was mostly asking about me dating other people, because my living situation is going from living alone to living with my girlfriend and her family. Her situation doesn't change much because she already lives with her husband. My concern, since I don't know how poly people feel about this, is about how someone I might try to date feel about me living with my girlfriend.
In a way, I am moving in to help with finances. I want to move in anyway, but their roommates are moving out, and they will have issues if they don't get someone to fill that spot. Of course, they could probably find another roommate. Also, I have a good deal on my living situation because my friend owns the place, but he can't leave it open, so if I move out, chances are it will be taken if I ever want to move back in. I can probably move back in with my parents if it comes down to it, which will help finances considerably if they don't charge rent, or move in with someone else, maybe.