Ian Wathey, 41, was imprisoned for five years, while his partner, Craig Faunch, 32, was sentenced to six years at Leeds Crown Court, for abusing four boys aged between eight and 14.
The couple from Pontefract, West Yorkshire, who had denied the allegations, were approved as foster carers by Wakefield Council in July 2003 but began using the boys for their own sexual gratification within months.
The court heard that Wakefield social services had been sent a photograph taken by the men of a boy urinating, which could have triggered an investigation, but that it had been lost by the department.
Judge Sally Cahill QC said neither of the men had shown any "empathy, remorse or any responsibility for their actions".
Faunch was found guilty of two charges of making indecent photos of a child and the court was told that he used a camcorder to film two naked eight-year-old boys in the shower.
He was also found guilty of five counts of sexual activity with a 14-year-old boy. Wathey was found guilty of four charges of sexual activity with another 14-year-old boy, and of encouraging a child to watch a gay pornographic video.
Referring to Faunch's abuse of a 14-year-old boy, she said: "You chose to victimise and abuse that boy, showing the very depths in my view of what you are prepared to do."
The court was told how the men had explained away the photograph of the boy urinating to social workers by saying that the images were taken to embarrass the boys into closing the toilet door. The photograph was sent to social services by the child's mother.
Social workers decided that the men had been "naive and silly" for taking the photo after hearing their explanation. Judge Cahill said: "Once you realised social services would not take any action ... you went on to abuse others in your care, believing yourself safe from the authorities."
The judge said it was "quite incredible" that police had not been involved earlier, which may have prevented further abuse.
After the men were convicted three weeks ago, Wakefield Council commissioned a full independent inquiry. Kitty Ferris, from Wakefield's social services, said the department had reviewed its internal procedures to identify what lessons should be learned.
"The allegations made against Mr Faunch and Mr Wathey were referred to the police as quickly as possible and were subject to a joint investigation by the police and social services under the safeguarding procedures," she said. "This investigation led to the prosecution of the couple. The council has offered support to children who have been affected by this case."
Neither man had been in trouble with the police before these incidents came to light, and were approved as foster carers after checks.
The victim whose complaint triggered the police inquiry gave evidence via a video link during the trial. Describing the abuse, he said: "It hurt. Afterwards, I said, 'Pack it in now,' and then I went to bed. I was gutted. I didn't want anything to do with anyone else. All I could do was sit there and cry."
Defence barristers for the two men had claimed that the allegations were "bizarre" and "incredible".
The rules
* About 50,000 children live with 37,000 foster carers in Britain.
As part of the shift away from institutionalised care, two thirds of children who are the responsibility of local authorities live with foster parents rather than in homes. But an extra 10,000 foster carers are needed to plug gaps in the service.
While the rules on gay men and lesbians adopting were changed only last year, they have been able to foster children for much longer.
All potential foster carers are subject to interviews with social workers, a home study and intimate questions about their lives. They are also asked to provide references and undergo checks with the Criminal Records Bureau and investigations into whether they have previously applied to look after children.
Once approved as foster carers, they undergo spot checks from social services and an annual review, while children under their care are visited by their social workers and talked to in confidence.
Experts said that the blame for yesterday's case did not lay with inadequate vetting procedures.
David Holmes, the chief executive of the British Association for Adoption and Fostering (BAAF), said: "The system that we have got at the moment is fine.
"The problem is that if somebody is absolutely determined to lie and deceive or try to undermine the system then they will do that.
"While it is impossible to prevent abuse from happening the important thing is to ensure that if something terrible does happen, the child feels able to disclose it to their social worker."
The blunders
Despite scores of reports and inquiries into individual cases, fatal mistakes have continued to be made in looking after children in care
* February 2000
Victoria Climbie, eight, died in Tottenham, north London, after months of neglect and abuse at the hands of her aunt Marie-Therese Kouao and her lover Carl Manning. They beat, whipped, burnt and starved Victoria; at the time of her death she had 126 separate injuries on her body. Social workers were criticised for believing what they were told by Kouao and considering her attitude towards Victoria to be a result of African cultural mores rather than plain abuse.
* May 2000
Six-year-old Lauren Wright was killed by her step-mother Tracy, who punched the girl so hard in the stomach that her digestive system collapsed.
Teachers, doctors and neighbours raised concerns about her welfare after seeing bruises. But social workers accepted explanations from Tracy Wright that the injuries happened in accidents.
Wright and Lauren's father, Craig, are in jail for manslaughter and neglect.
* September 2003
Toni-Ann Byfield, seven, was shot dead in London where she stayed with Bertram Byfield, who was then thought to be her father. Social workers gave permission for her to stay with a woman in London who told them she was Toni-Ann's aunt. No checks were made and the woman was the girlfriend of Byfield, a crack dealer. He and Toni-Ann were shot in what is believed to have been a revenge attack for a bad deal.
