Like the vore FPS, I don't understand having a fetish extend outside of masturbation. Does the fetish spread outwards from the arousal center of your brain and infect your fun and humor centers too?
I think it's people desperately trying to convince themselves there's nothing wrong with their fetishes, so by externalizing them as animations, fanfic, whatever they create an insulation of "hobby" and community to make them feel more at ease with the fact they get off on chicks being eaten by sharks or whatever.