Of all the weird creepy sex shit common in manga, somehow the idea you can tell if a person is a virgin by looking at their hymen is still really creepy sex shit
He wasn't using it. It was already established with the duel that they can detect if magic is used so they would know if he was using magic in the past.
I kind of wish this would go with him deciding she likes sadism so playing along with it in a way that ends up infuriating her because he isn't actually getting wrecked