The 14 year-old boy was babysitting the girl when he attacked her, a court heard.
But Judge Gareth Hawkesworth let him walk free because he said the schoolboy, who regularly accessed hardcore pornography on his laptop, was just being impulsive.
The judge told him: "I believe you have become sexualised by your exposure to and the corruption of pornography. Your exposure at such a young age has ended in tragedy.
"It was the fault of the world and society."
But the judge's decision – and his reasons – caused fury amongst radio phone-in callers.
One said: "If a teenager steals a car and crashes it into a bus queue, do you blame the car?"
Others said the boy's parents should be held to account for not supervising his access to online pornography.
The decision not to imprison the boy came as the judge admitted he would have got six and a half years if he had been 18. Instead he was handed a three-year community order.
The youngster was paid £10 to babysit at the girl's home.
He admitted the rape after his victim told her horrified father.
Julia Flanagan, defending, told Cambridge Crown Court: "He was watching excessive pornography, which was very inappropriate for his age. We have a very frightened and remorseful boy who is very ashamed. This was an isolated incident. There was no threat or force used."
The boy, who is now 15 told police he had "lost his mind".
He added: "I think my hormones took over."
The judge said: "I'm satisfied it was impulsive."
He imposed a five-year sexual offences prevention order - barring the boy from using computers or mobile devices to view porn.
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What do you think? Do you have parental controls on your laptops and computers?