It's the fingernails at his back that really spurn him forward, it makes him sharp and attentive and draws more breath from him than it should. Her murmuring isn't a bad incentive either, though.

He pulls his hand away, biting at her lower lip. It's not enough to draw blood (not yet, anyway), but he means it to be a suitable distraction while he feels around for the top of her panties and tugs them out of the way. He runs his fingers over her a little, just enough to get her used to the idea, before he slips one, two inside of her and feels his way around, trying to find a response.