can I interject a bit in here too please?
what I'm mainly concerned over in this is it sounds to me like your fella is interested in you and this other gal hitting it off come hell or high water, and there is so many issues with that. Firstly, its YOU whom decides who you choose to have a relationship with, not your SO. Secondly, why can't you two just be friends? If he likes her, great, why does that mean you have to? and another thing, if he wants a relationship with her, why does that necessarily mean she gets to move in?
It seems to me that he really needs to back off and let you and this other gal meet and form whatever you will, be it freindship, something more or even something less. And unless you both had some previous aggreement stating someone else would eventually be joining your relationship and moving in, I think that is simply way tooo fast and too early of thinking to be having.
Sit him down, talk to him. Sounds like you wouldn't mind him being poly, but you don't wanna live with the other person and you don't wanna forced to love them as well. Nothing wrong with that. or if I totally misread what you said there, and it is him being poly that you mind, you may have to let him go. It seems he's quite intot his other person if he's all wanting them to move in. Talk it out, make sure everyone is saying exactly what they mean, decide what exactly you do and don't want, and don't let him assume things are okay. Maybe he simply meant eventually when you find the right person he wants them in your lifes totally, eventually he wants you and the other person to be involved too. Go talk.