She isn't thrilled with that idea

 "That's... really personal," she blushes.  "I wasn't expecting that at all."

 "You did promise."

 "Yeah, but..."

 You wonder what the reason for her hesitation could be.  She clearly isn't comfortable with the idea, but her **** just makes you all the more curious.  The image of your shy girlfriend with her legs splayed and rubbing her tender clitoris is something that refuses to leave your mind.

 Your cock grows even harder, and you realize that there may be one way to encourage her to open up.  Without a great deal of fuss, you undo the top of your pants and casually extract your erection.  She eyes grow wide at the brazen sight of your cock.

 "What if I start off first," you suggest, softly stroking your length in your hand.  Her eyes are rivited to the spot, her expression a combination of horror and fascination at your act of self pleasuring.

 "Fair is fair," you say, motioning that it might be her turn to join in the fun.