Comment by matthijs

Even a really close up picture of a face will be seen as nudity. Trying to detect a body (or parts of a body) and determine how much skin is visible would probably be a better approach.

Also quite relevant is this site: http://www.yangsky.com/researches/physicallinguistics/PLUnde... Besides a "breast detector/nipple detector" he also created a couple of weird detectors like a "cowgirl sex position detector" among others. (The link is sfw, the individual detectors are not)