A question about sexting.

CNN reports:

Phillip Alpert found out the hard way. He had just turned 18 when he sent a naked photo of his 16-year-old girlfriend, a photo she had taken and sent him, to dozens of her friends and family after an argument. The high school sweethearts had been dating for almost 2½ years. "It was a stupid thing I did because I was upset and tired and it was the middle of the night and I was an immature kid," says Alpert.

Orlando, Florida, police didn't see it that way. Alpert was arrested and charged with sending child pornography, a felony to which he pleaded no contest but was later convicted. He was sentenced to five years probation and required by Florida law to register as a sex offender.

"You will find me on the registered sex offender list next to people who have raped children, molested kids, things like that, because I sent child pornography," says Alpert in disbelief, explaining, "You think child pornography, you think 6-year-old, 3-year-old little kids who can't think for themselves, who are taken advantage of. That really wasn't the case."

My question: When did nudity become pornography?