Porn is a Massive, MASSIVE business. It has it's own industry infrastructure/supply chain, banking, industry award system and critical review and with that come its "connoisseurs". Interviews with employees who've been caught with porn on company resources show there's software for organizing, categorizing and hiding the stuff, some of the more hard-core stuff was collected because at some point, the individual just couldn't enjoy or "get-off" on regular old porn anymore. They needed something "different" or more "exotic" to satisfy. Weirdest porn stash I've ever seen hidden on company resources was stored on a Xerox printer's memory via the on-board ftp server.
My first job out of school was with a start-up ISP and I worked nights where this one guy would always come in every month at 10PM to pay for his co-lo sever with cash. He ran a website that specialized in BBW (big beautiful woman) porn. It was his only source of income and at the time, '95 he said he was making around $70,000 a year off of it. Evidently the speciality porn: gay, tranny, scat, BBW, etc. is where the money is.