The Instagram message popped up from a girl named Chelsea: “Howdy.” David didn’t know anyone named Chelsea, but he clicked through her profile: She had brown hair and a nice smile; under her name was a quote from the Bible. He thought it was sort of weird that she was messaging him, a stranger, in the middle of a workday, but her pouty selfies made that easy to ignore. He was hesitant when she asked him to chat, but soon her flirty messages escalated to a volley of explicit pictures, and David, a 32-year-old pharmacy technician, got carried away.