The very tips of his fingers brushed her underpants feeling dampness - dampness mirrored in his own underwear. In that moment, he made a decision, pulling his wand out of his pocket and pointing it at his crotch, muttering quietly under his breath.
The usually-soft packer he used to simulate the effect of having a penis swelled, stiffening in much the same way a real one would. His jeans became tight to the point of being uncomfortable, but he welcomed the sensation. It was normality. He was a teenage boy in bed with a pretty, half naked girl. It was natural that the sights, sounds and sensations would make him erect.
He looked up at her, then put his hand back up where it had been. His breath was coming hard and fast, but he soldiered on, maintaining eye contact as he pulled her underwear to the side and touched her bare flesh.