Some excellent links with this regard on Wikipedia.
http://en.wikipedia.org/wiki/Obscene
Even with regards to the recent case with Red Rose stories... Direct quote
"Also, "it appears the Porn Squad has been told that the best possibility of prosecution includes golden showers, scat ... and BDSM along with other fringe fetishes... [the US] government is not targeting kiddie porn only"...
Think again about those legalities, kiddies.