Maybe just the thought of transgressing by topping you is erotic for her.
Not maybe, that's exactly it. There is something very erotic to see a strong man's body, the shirt off, hands tied loosely behind the back. A whip is playing over his back, lightly in the beginning, then more and more strongly. Stop for a pull of his hair and massage of his crotch, then some more whips and watch his muscles strain and move. Then the sarcastic look thrown over his shoulder , as if saying "This is all you can?", then his words "NOW, untie me" ....

Something like this. Nothing humiliating at all, just erotic and sensuous.