Ms Rantzen had a short relationship with Mr Fairbairn, the former Conservative MP for Kinross and Western Perthshire, when she was 26, after they met at the BBC in 1966.
She said: "I am horrified and disgusted by these allegations, because Nicky was a friend of mine. I had a brief relationship with him. I always assumed that he was attracted to adult women rather than children."
She was speaking after details emerged suggesting Mr Fairbairn, who died in 1995 aged 61, had visited a London guest house where children from care homes were known to have been assaulted by visitors, including senior establishment figures.
Police are investigating parties in the Elm Guest House, in the 1980s where vulnerable boys were given drink and sexually assaulted, as part of the Operation Fernbridge inquiry. Reports suggest that documents held by the police list N Fairburn, along with C Smith as having visited the property in 1982.
Fairburn, despite the misspelling, is believed to refer to the late MP, while the other name is that of Cyril Smith, now know to have been a serial abuser.
The documents also state that "Fairburn" had "used" boys in the sauna.
Ms Rantzen added: "I had no knowledge of that side of him. However, over the years I have learned that you really never know anyone.
"When I knew Nicky he was courteous, charming and very fond of women. He was a high-profile lawyer, and had a very flamboyant private life.'
The presenter said victims of abuse needed justice. She said: "Child abuse has always been a crime and, in my experience, there was never a time when it was tolerated."