Originally Posted by Wikipedia
Inhaling nitrous oxide can cause analgesia, euphoria, dizziness, flanging of sound, and slight hallucinations.
Although it seems silly, could you have been hallucinating that he violated you? I also have my sincere doubts that you would be unconscious or incapacitated to a point where you would be either unaware or unable to fight back against the dentist. Also, I am extremely surprised that he didn't appear to make even the most rudimentary efforts to hide what you claimed he had done.