I wouldn't worry about who she's fantasizing about (since a fantasy is just a fantasy), but perhaps she needs a little more "aggressiveness" and interest from you. Go to the sex section in your local book store (usually near the relationship books) and start reading. Maybe try some new things with her. Ask her to tell you exactly where and what she needs. Say something dirty in her ear....
I wouldn't bring up the masturbation, but instead consider how you might make sex better for her.
There is no arriving, ever. It is all a continual becoming.
