James Rennie had been keen to babysit for one couple
A trusted babysitter recorded his abuse of a young child and shared the images with friends and anonymous internet contacts.
The betrayal emerged during a trial at the High Court in Edinburgh, which ended with eight men being found guilty of a variety of charges including child abuse, conspiracy to abuse children and possession of indecent images.
The parents of the child involved had no idea what was happening, until detectives told them about photographs and video material they had seized.
One couple told how their lives were changed forever by the betrayal of James Rennie, who had been babysitting since their son was three months old.
They had to have their son tested for HIV and other sexually transmitted diseases.
The youngster's 37-year-old mother, who cannot be identified for legal reasons, told the trial: "I think it would be fair to say it has turned our lives upside down.
"For the last year, trying to care for our son in a normal way has been very challenging.
"Trying to live and sustain relationships at work and with friends has been very, very difficult."
Her husband, also 37, said Rennie always seemed keen to babysit and spoke of the boy as his nephew. There were presents at Christmas.
When 38-year-old Rennie confided that he was gay, the couple were "very supportive".
Rennie, the chief executive of LGBT Youth Scotland, an organisation which offers advice on sexuality to young people, was trusted to bath the baby, change nappies and put him to bed.
The father said he was angry and confused after seeing the images. The court heard that there could be no mistaking the child's clothing, his "little pot belly" or the colour of the floor.
"That has been a thing which has affected every day of my life and it has affected every relationship I have, whether it be with my colleagues at work, my friends, my immediate family, with (my wife) especially and of course with (my son)," he said.
He said he had been "terrified and exhausted" watching his son go through all the medical tests.
LGBT Scotland also felt a betrayal of the ideals of the organisation, and gave police full co-operation.
Neil Strachan was pictured raping an 18-month-old child
One other key piece of evidence in the trial was described as "the Hogmanay Image", which showed engineer Neil Strachan raping an 18-month-old child.
Electronic data showed that the photograph had been taken an hour before midnight on 31 December 2005.
The toddler's father said he did not want to see the photo when police brought it to his Fife home.
Asked to sum up the family's ordeal, the 33-year-old said: "Words cannot describe it, words cannot describe it."
He said he and his partner felt traumatised and angry. "Everything. You name it - we felt it."
The images were shown to the jury of eight men and seven women, along with 40 other photographs.
The trial also saw records of hours of internet chats spanning more than four years which featured descriptions of sadistic sex.
One description of the rape of a girl even caused judge Lord Bannatyne to abruptly adjourn proceedings for a few minutes, without giving any explanation.
A detective with years of experience had difficulty describing his feelings as he gave an account of a four-year-old girl's protests as she is made to perform a sex act.
The images, among almost 125,000 uncovered by police, were unlocked from encrypted computer files after months of work by experts.
"All I can say is there are certain images which will make the hair on the back of your neck stand up," said Det Insp David Reid.
He added that he and his colleagues had received counselling sessions every six months to help them cope.