But after he left, the girl described to her father how she had been subjected to the oral rape after the teenager told her to cover her eyes.
Mr Kellett said: "The father thought he must have misheard and she repeated it."
The father told police what had happened and the girl repeated her story to officers the following day.
The teenager admitted the offence to police and told officers that it happened when he had "lost his mind and didn't know what he was doing".
He added: "I think my hormones took over."
Mr Kellett said the boy admitted he regularly looked at pornography on his laptop at home.
The court heard the victim's father was very concerned about the long-term psychological damage caused to his daughter by the traumatic experience.
Julia Flanagan, defending, said the boy knew how his actions had impacted on his family, specifically his mother, who had given up her job as a childminder.
She said: "This was an isolated incident. There was no threat or force used and he stopped immediately.
"He is an isolated and solitary child. He was watching excessive pornography which was very inappropriate for his age.
"We have a very frightened and remorseful boy who is very ashamed of what he has done. 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."
Judge Hawkesworth warned the boy that he would have been jailed for six and a half years if he had been four years older, but he instead gave him the three-year community order with a supervision requirement.
The judge told him: "You have not shown any partial 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 judge requested six monthly reports be sent to him to monitor to the boy's progress and ordered him to register his whereabouts with the police for two and a half years.
A sexual offences prevention order was also made for five years, stating he must not access or seek to access pornography of any kind
The youth was also banned from using any mobile devices or computers without suitable filters to prevent the viewing of pornography.
