The Who guitarist and songwriter was cautioned and put on the sex offenders register for five years in 2003 for looking at child porn on the internet.
The star insisted he was looking at the website only while conducting research for a campaign against internet porn involving children.
He has now given more information in an interview with a newspaper which is serialising his memoir, Who I Am.
He described his actions as a product of "white knight syndrome, an attempt to be seen to be the one that's helping".
Speaking from his home in Richmond, south-west London, he said: "It's a product of success. I had experienced something creepy as a child, so you imagine, what if I was a girl of nine or 10 and my uncle had raped me every week? I felt I had an understanding, and I could help."
He intended to show child abuse has a financial chain that runs from Russian orphanages to British banks by paying a £7 charge for a child pornography site, which he cancelled immediately, the newspaper said.
When police subsequently confiscated his computers and files they found nothing incriminating, but the damage to his image had been done.
"What I did was insane," said Townshend.
Asked why he did not speak out sooner, he said: "Because there was no sense of 'the truth will out'. I've had the misfortune to read online comments where I'm judged as a paedophile because I've got a big nose."