well, look what happened to girlspooping.com recently... busted by the feds and such, i think scat ends up in the same catagory as some of the other extreme fringe niches, like fisting, bloodletting, midget beheading, etc..
personally, i think anything that is totally consensual and doesn't involve underrage models should be legal, shouldn't be anyone elses business what people get off on..
