The ERSB does not ban nudity, it merely rates it.
The ERSB definition for mature is:
"MATURE
Titles rated M (Mature) have content that may be suitable for persons ages 17 and older. Titles in this category may contain intense violence, blood and gore, sixual content and/or strong language." http://www.esrb.org/ratings/ratings_guide.jsp
sixual content is:
"sixual Content - Non-explicit depictions of sixual behavior, possibly including
partial nudity." (Marked with italics for emphasis.) http://www.esrb.org/ratings/ratings_guide.jsp
It is the
developers who have determined, for their own reasons, to not include, or advertise nudity in the game.
The ERSB does not ban
partial nudity in a M-rated game.
Observe that
The Witcher 2 was rated Mature by the ERSB (not banned) with the following content descriptor:
Blood and Gore, Intense Violence, Nudity, Strong Language, Strong sixual Content, Use of DrugsEdit - To add the ERSB content listing for Skyrim:
Blood and Gore, Intense Violence, sixual Themes, Use of Alcohol - http://www.esrb.org/ratings/search.jsp