From Wikipedia

The Nude In Art

Episode Three

Nudity in art, also publicly displayed, is rather common and more accepted than public nudity of real people. For example, a statue or painting representing a nude person may be displayed in public places where actual nudity is not allowed. However, there is also much art depicting a nude person with a piece of cloth or other object seemingly by chance covering the genitals. Some feel the selected focus of "Nude studies" lends itself to an impersonal, objectifying depiction of the human body; others say it can be as selectively depicted as a landscape.

A 1960s comedy sketch featuring English comedians Peter Cook and Dudley Moore admiring Cézanne's Les Grandes Baigneuses in the National Gallery humorously suggested that there must be hundreds of paintings that are not publicly displayed because the pieces of cloth did not fall in just the right places while the artist was painting them.