Not that I've met her but don't be so certain. A lot of 'good girls' will be getting it on with f*ck buddies.

As for stating your intentions. Do it. You are right to say that being just friends leads to misery. Tell her what you want. It's the least you should do as she has done just this.

I would also add that personally I would take things as just fun to begin with. I like to get to know a girl not only in the bedroom but as a person. I like to be sure before stating to a commitment. Maybe this is what she is thinking as well?