Rose couldn't help but give a ridiculous smile when he said it. Every time. It was a wonder she didn't giggle. She nuzzled his neck, blushing... which only deepened when she felt him pressing against her leg. She suppressed a little 'eep' of surprise by shoving her lips against his neck.
Really, it was silly for her to be surprised. It wasn't as though Rose was a blushing virgin. But to think about the boys she'd dated being horny and the Doctor being horny were two entirely different things.
Focus, Rose. She'd discovered a lovely little spot there on his neck, and she wasn't about to give that up. Doctors were all about science, and science was all about experimentation, wasn't it? She wondered what would happen if she gave that little spot a nibble.