Alexa is 15 now, but she first heard about it when she was 10. She remembers thinking, "Ewww." Disgusting. The next time the subject came up, she was 13, an eighth grader at an affluent private school in Toronto, making out with her boyfriend in his bedroom. He asked if she was willing. "He told me, 'You don't have to if you don't want to. It's up to you.' But I really liked him and this was my first, like, obsessive thing with a guy." He was a year older, 14. "I wanted to impress him. And we'd been hooking...