anony,
I will start by saying that I realize I am "sticking my neck out" here in saying this...but, there is a reason that sex was reserved for marriage. It creates soul-ties to another person, which definitely do exert a measure of control, as you stated about feeling "powerless." Sex is just one way a somatic narcissist uses to control another person.
In my view of this, the lady is wrong for using you and then only calling you "friend." That is extremely misleading. Your part in this, however, is in the allowing her to have your body in the first place. I am a bit old-fashioned and simple when it comes to some things, and, in this case, I'd like to introduce you to a friend of mine...his name is "NO" That word, when applied and stuck to, goes a long way in self-protection and warding off unwanted affections.
She's using her and you are letting her use you. A codependency group for you might be very helpful in this. I still use my codep book often as well. The relationship is not a healthy one but a very toxic one.
Sorry if I'm being too blunt. When it comes to N's I not only know what I'm talking about from experience, but I have very little tolerance of their abusing others, as well as the others continually opening the door and saying "come on in to abuse me and then devalue and discard me when you're done." We, as the victims have a responsiblity to also protect ourselves.
~RM