>nor do I really understand this fascination you people have with deviancy
It's a way of making life a bit more fun, but it's also intereresting to see how people reply to these questions, what they reveal about themselves through them, what they're afraid of, what they're proud of having done, how they're similar or different from you in the way they read into questions and analyze them, etc. You can call it stealth datamining, but I prefer thinking of it as a way of getting closer to other people. It's also way of reassuring the idea that there's always gonna be somebody who might be weirder than me (as well as people who aren't).
Why specifically sexual topics? Because that's what people on /bant/ (and 4chan, maybe even on all of the Internet) get most excited about.