I can’t tell you how many times I’ve opened my email to find an unsolicited photo of a man’s penis. No, I’m not a “friend” of Anthony Weiner. I’m a journalist who delved into the everyday world of online sex in order to research the sex practices of average Americans – not the powerful or the rich or the famous. I’m talking the neighbors next door – Joe and Jane Schmo.

And if there’s one thing I discovered in nearly eight years of talking to people about their sex lives, it’s that a whole lot of Americans are taking pictures of their genitals and proudly posting them for others to see.

Within two days of beginning my research in 2004, I received an email from a man in his thirties whom I'll call Jim. Jim had answered my Craigslist casual encounters ad seeking men and women willing to tell me the intimate details o...