Ask nicely

"Please. You've seen me."

You gesture at your raging erection. To help seal the deal you stand up and drop the robe entirely. You kick your boxer shorts out of the way and stand there in all your naked glory. She stands there with her arms over her nipples, her fingers holding the edge of her panties. She looks at you. You can see her still wavering. She starts to lower her panties and you get the tiniest glimpse of her pubic hair, then she gets frightened and covers up again.

"Please?"