Shock value comes from dissonance, and in any sort of fandom the most shocking things acquire a kind of perverse (for want of a better word) fame. It's the reason for such things as Dipper Goes To Taco Bell, Cupcakes, and certain Harry Potter fanfics that shall remain nameless. It's also why there exists a Friendship Is Magic crossover with 120 Days Of Sodom. You'll forgive my reluctance to link to any of these.

Of course, there are those who genuinely do get off on this, particularly in Japan - and I'd put money on it being down to the censorship of more, ahem, conventional spank fodder. The tentacle thing, for instance, likely came about because you don't have to pixelate a tentacle when you must pixelate a throbbing purple-helmeted man-carrot. It's a vicious circle, too; people who are a little bit into the weird stuff get drawn deeper into the rabbit hole, and before you know it you're looking at bandaged stumps and so on.

I don't have a sociology degree, but I am quite monstrously pretentious, which has to count for something. =]

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"She's old, she's lame, she's barren too, // "She's not worth feed or hay, // "But I'll give her this," - he blew smoke at me - // "She was something in her day." -- Garnet Rogers, Small Victory

FiMFiction.net: we might accept blatant porn involving the cast of My Little Pony but as God is my witness we have standards.