The more long tail and explicit your search the higher your probability for sex images becomes. Broad searches will not show nudity, normally.
I.e. try entering a name for a famous adult actress into Google's image search. One that has been around for a couple of years like Tera Patrick or Amber Lynn. Most likely all you get are portrait or bikini photos. Only when you narrow your search to "Tera Patrick boobs" or "Amber Lynn sex" you will receive nudity.
Notice how well "Tera Patrick boobs" displays boob pictures, only. On the contrary less explicit searches like "tera patrick ass" or "tera patrick gay" don't reveal any explicit shots again. It seems Google puts a lot of effort into being exact where it wants to show nudity, but turning away from skin when searches are broad.
BTW, here is my original post:
http://www.megamasters.com/google-im...rch-censorship
After I wrote this article I did receive a little bit of image search traffic for "anal sex", indeed.