With any sexual activity it's not realistic to expect that it's going to be just like your fantasies.  Pegging porn is hot sure...but it's scripted.  Dominance and role reversal is not always going come naturally to every girl.  She may crave a dominant man herself and you've just cast yourself in a different light for her.  It can be an adjustment so you always need to approach it with patience and understanding.  Talk to her about any concerns she has about taking the lead.  Is she worried you will always want it?  Is she afraid where it will lead?  Those are valid concerns that you need to work through with her.  Reassure her you will still be the man she needs sexually and that pegging is just a fantasy not a lifestyle change.  It's unrealistic expect her to take on the dominant role right out of the gate AND that it will be a turn on for her.