"If the act is treated as rape, he should be punished for rape. It sends the wrong message to people - that you can do something like that and get such a light punishment.
"And it could stop victims coming forward. What's the point in going through all that?"
The girl's father, a businessman who cannot be named for legal reasons, told the Sun he had trusted the teenager to look after his daughter, who was just four when it happened, while they watched her older sibling in a school play.
He said they were "just feeling completely devastated" after the little girl told them what had happened at her bedtime.
Her attacker, now 15, told police he "lost his mind" because his "hormones took over". He told officers he had regularly viewed "excessive" pornography on the internet via a laptop at home.
His parents were so appalled by the offence that his mother left her job as a childminder.
At Cambridge Crown Court, Judge Gareth Hawkesworth said exposure to adult pornography had led to the incident and blamed "society".
The boy - who wore his school uniform in court - was told he had only been spared six-and-ahalf years in jail because he was a minor. He received a three-year community order with a supervision requirement.
The judge said: "You have not shown any sexual interest in children.
"I'm satisfied it was impulsive and 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."
The teenager, who cannot be named, pleaded guilty to raping a girl under 13 at her home in Cambridge in December last year.
John Kellett, prosecuting, said the girl spoke to her father after the boy left, describing how she had been subjected to oral rape. "The father thought he must have misheard and she repeated it."
Julia Flagan, mitigating, said: "He is an isolated and solitary child. He was watching excessive pornography, which was very inappropriate for his age."
She said her client was frightened, remorseful and ashamed. "He knows that what he has done will follow him for the rest of his life. He is marked to some as a sexual deviant forever."
The judge asked to see reports on the boy's progress and ordered him to register his whereabouts with police for two and a half years.
A five-year sexual offences prevention order states he must not seek access to pornography of any kind or use any mobile device or computer without filters to block pornography.
Claire Perry, a Conservative MP and leader of an all-party group campaigning against open access to pornography, yesterday said the "harrowing" case emphasised "that pornography can have serious negative impacts on children's lives".
Jon Brown, of the NSPCC, said it wanted a system where adults had to actively opt-in to view internet pornography.
"Hopefully the young victim who suffered this awful ordeal will get the therapy she needs and the offender undergoes a course of treatment so he fully understands the impact of what he has done and never repeats his actions," he said.
