Why is Penny naked?

She violated one of Sheldon's rules

''You signed the contract yourself, Penny. Now get out of there.'' She could hear the brilliant, but socially impaired scientist Doctor Sheldon Cooper calmly explain through the closed door of her apartment. While it was true that she had broken one of Sheldon's ridiculous rules, she didn't quite understand why her punishment differed from the boys'.

''Can't I just sign up for one of your classes, like Wolowitz?'' The blonde inquired, but the answer was no different than it had been on her last sixteen attempts. ''At the meeting of the 13th of June, which you did not attend, we took a vote about reviewing your agreement. You had a chance to read it all, but it seems like you didn't do that and now there are consequences. There's nothing I can do for you, Penny.''

24 hours of nudity... Penny sighed. She surrendered. She unlocked the door of her apartment and opened the door, and there she was.. in full glory. Even though she tried to cover her curves with one hand, they were too big to leave anything to the imagination still. A landing-strip adorned her nether region. Sheldon, usually unbothered by carnal desires, found himself awkwardly yet surreptitiously tugging his hard-on to the side, obscuring it from the blonde's view.

''That's better.'' Sheldon stated, as if this was not the most exciting thing he'd ever laid his eyes upon. ''Are you hungry? It's Wednesday, which is pizza day.''