Personally, I think that's the real reason they're so erotic. They invite us to trust deeply, to open ourselves to a surrender that doesn't feel safe in the regular world. But with the rules and agreements of sexual play acting, we're invited to surrender; to pretend we have no control over the situation. We give ourselves permission to open ourselves fully in love. It's incredibly erotic, and terrifying to many because of this opening.
Excuse me, I need to go find my hand cuffs. (If you want to read more of my stuff, you can go here).