I don’t mean to argue, but I think TChad’s explanation is the correct one.
Did you read the article that I linked? I was following those Lafave threads from the beginning and can assure you that's how it started. It has taken on a life of its own now, but at its core it's sarcasm related to the Lafave case. Sure, there are often discussions in those threads about how it's a victimless crime since for most teen boys (me included when I was a teen) it would be a dream come true to have sex with a moderately attractive teacher, but that's not what the whole guilty/not guilty thing is about.