From the YouTube Guidelines Page...
Most nudity is not allowed, particularly if it is in a sexual context. Generally if a video is intended to be sexually provocative, it is less likely to be acceptable for YouTube. There are exceptions for some educational, documentary, scientific, and artistic content, but only if that is the sole purpose of the video and it is not gratuitously graphic. For example, a documentary on breast cancer would be appropriate, but posting clips out of context from the documentary might not be.
More info here...
https://youtube.com/t/community_guidelines