Lucy pulls them both up against the rock wall, so that it can take some of her weight as she pulls up her skirt and Peter lifts her, her legs wrapping around his hips. One arm's around his neck, and the other's reaching down between them to help guide him inside her. She moans into his mouth as he pushes inside.

This would probably be a lot more strenuous and uncomfortable if this wasn't a dream, but as it is it just happens to be really fun.