At this point you cannont move in together. At this point someone else has got her interest and attention. As a guy you know that we seldom see things as just friends and on the odd chance your lady does see it as just friends I think we all know the guy is most likley wanting to get into your girls pants.
I have not read this entire thread but if you have been together long enough to move in it must have been a serious relationship. I would think she would understand your concerns, if she does not it just further shows she is not the right chick for you.
Tough situation for you and I am sure there could be heartbreak but better to find out now then to be stuck in a lease or something with her.