He hadn't meant to spy on her. Really, he hadn't.
He had finally left his room after tossing and turning for hours. He had almost passed the slightly ajar bathroom door, but stopped when he heard the soft sound of water dripping.
Somewhere, in the back of his mind, Harry had always known Hermione was a girl. However, knowing was a far cry from having that fact driven home by the sight of her covered in nothing but water. Even though he couldn't really see anything, he became uncomfortably aware of how vivid his imagination was. He could see her, in his mind, rising up from the bath and walking toward him.
Granted, he had never seen a naked woman live and in person. However, the Weasley twins had made sure he had a modest supply of girly mags, and he was a normal enough boy to have looked them over at least once… maybe more than once.
He was slightly roused from his imaginings when Hermione started to move. He saw the leg nearest him come up and rest on the edge of the bath, and her hands disappear from his view. Her mouth opened slightly and her tongue lightly touching her top lip. It took the arching of her neck and a slight moan for Harry to catch on to what was happening.
Hermione was wanking. Or, whatever it was girls called it. She was doing it right there in the same house as him.
Eyes wide, Harry backed away as quietly as he could, then rushed off to his room. Heart pounding, he dove into his bed, yanked the covers up to his chin, and tried to get the image of Hermione out of his mind.
Finally, giving up and trying to ignore the guilt in the pit of his stomach, he bit his lip, squeezed his eyes shut and shoved his hands down his pajamas. It was far too easy to imagine her in the bath, head thrown back and moaning. It was quickly over the minute he imagined her saying 'oh, Harry' in a breathy voice.
After cleaning himself up, he lay in his bed and found himself wondering how long it would be before he could look at her without being uncomfortably reminded of tonight. He really hoped she never looked below his waist… ever.