"My mum literally said she was shocked," he told the veteran British journalist. "My friends were a bit more like, okay, fair enough. They said they had some suspicions."
But Thorpe said he didn't want anyone to go through his pain.
"I don't want young people to feel the same way I did," he said. "You can grow up and you can be comfortable and you can be gay."
The 31-year-old's admission, which prompted an outpouring of public support, followed a long stint in rehab confronting personal demons that drove him to the verge of suicide.
Thorpe spoke candidly about his battles with alcohol abuse and crippling depression from an early age.
He contemplated suicide, but the likely effect on family and friends pulled him back from the brink.
"I couldn't do it to them," he said. "It was the only thing that stopped me, I think."
Thorpe's unusually large hands and size 17 feet propelled him to five Olympic golds, 11 world titles and 22 world records during a glittering career.
He said he was "a little bit" ashamed of not having the courage to "break the lie" earlier, admitting it would have made his life easier.
He said he had repeatedly denied he was gay because he was scared of people's reaction.
"Part of me didn't know if Australia wanted its champion to be gay," he told Parkinson.
Thorpe says his only sexual experiences to date have been with women, but he would like to find a partner and raise a family.
"I never really considered hooking up with [men] because I was scared of people finding out," he said.
"I was trying to avoid all of this."
The interview was filmed in England last month following Thorpe's emergence from rehab. The swimmer is reported to have been paid A$400,000, with Parkinson pocketing around A$100,000.
