there is also the view that (especially in light of the 'women like you' comment) that maybe she doesn't feel quite so open and upfront about what she is doing. anything out of the conventional sexual 'norm' is tainted with a certain stigma. Maybe she can't let herself call you her 'girlfriend'.
I also don't think we can ever assume that everyone feels the same need for honesty that we do.
There is also the thought in my head that maybe she purposely keeps you a bit distant so she can engage in other relationships.