Being half naked makes Michael insanely unsure of himself. No one has ever seen this much of him except his dad and other boys in the locker room at school when he was a kid. He doesn't have intense self-image issues, but he doesn't have any self-esteem either, and he's suddenly suspicious that Lee will find something wrong and hideous about him.
On the positive side, his worry has him freshly excitable. He feels tender and prickly beneath Lee's hand, can feel the fabric of her shirt and the heat of her torso in a very immediate way. One of his hands goes to her breastbone and the other rests against her back, thumb tracing along her spine. He's trying to ignore the fact that he's getting hard, because that got him in trouble last time they tried this, but he's breathing a little heavier and his tongue is in Lee's mouth and he's really not sure what to do with himself.