I'm one of those people who have always known on some level, even before I knew the terminology. When I was young enough to be reading the Black Stallion books (maybe 8 or 9), I would pretend that I'd been abducted, held prisoner, and tortured. I would even tie myself up in bed. I would read any children's/young adults lit that had any measure of pain or punishment in it, and imagine myself in the character's place. When I was 16, I came across a copy of Exit to Eden at a garage sale, and devoured it in one night. It was like the author (Anne Rice) had guessed all of the fantasies I'd never even known I had, and wrote them down. I've never been able to be part of a 'normal' relationship since.