It boggles my mind that they seem to place so much emphasis on whether it was over the clothes or under.
Regardless, it's gross and he was old enough to know better. It wasn't like he was really young, he was basically 15 at the time.
The Duggars are trash.
EDIT: I also highly doubt that when he 'fessed up' to his parents that he told the real story. He likely hid details about what he did, and made it seem better than what it was.