EXCERPT: By today's standards, the high school girl from Montgomery County was a slow starter when it came to sex, having reached ninth grade before she kissed a guy. She just couldn't see how to fit a boy into heavy loads of honors homework, soccer matches and baby-sitting.... By 11th grade, she had found the answer: "buddysex," or sexual encounters with friends, in this case a half-dozen private-school boys, no strings attached... "September through December is a blur," she recalls as winter sunlight glints off her shoulder-length blond hair and warms her Cafe Mocha. "Let's see, I hooked up with...