Put a bit more gently: In order for a romantic connection to happen, a physical one usually needs established first.
It doesn't have to be a one night stand, but you don't even really know this girl. How do you just know you want an exclusive relationship with her? If your honest with yourself, you'll see that thinking this way is kind of silly.
The right answer is to date her, and when the opportunity arrives sex her. If you can stimulate her sensually, she will push a more commited relationship on her own. While you're waiting for all of this, consider that shes at least mildly interested in you - she wouldnt have given you her number if she wasnt. Have fun, and evaluate her along the way. That way, by the time she wants to take it further, you'll be able to give her a decision that is honest to both of you instead of winding up obsessing over how to keep her.
Good luck.
P.S. Put a little more crudely: Beat them cheeks real good, and you wont have to bring up having a relationship, shell beg you for one.
I have a feeling that how you view the process of human relationships is a bit warped. Check this out: On a biological level, we were designed to have sex. We were never designed for monogamy. Its a practice, not an instinct.
Right now, I think your seeing relationships as: Get number -> Go on dates -> "Serious"/"In Love" -> Sex -> Marriage/Have kids/Whatever
The truth is that as a species we aren't wired that way. The simple reality is: Meet girl-> attract girl -> sex girl. And thats where the natural process stops, and the artificial one begins. If you feel like shes worth your time, continue sexing her. If she is interested, she will provide signs for you to make a move all the way up to marriage should you choose to stay invested that long.
