They say being gay is a sin – and they can't get over it

David Pickup, a "reparative therapist" from California, runs some slides off a projector. He shows a happy, portly, gay couple in a posed photo. "On the outside they might smile, but inside they are numb or dead. Gays that are happy are gays who don't know about themselves," he says.

A series of images appears on the wall: a small child crying in a dark corner, someone self-harming with a kitchen knife, a man lying dead in a coffin. This is how gay people come to him, apparently, when they see him for counselling in Los Angeles. These are the broken people God told David Pickup he must repair.

David Pickup, who runs therapy sessions to turn gay men into heterosexuals, was speaking at an event in London in June last year. It was held at the Christian Legal Centre just off Bond Street, and was organised by Core Issues, whose adverts were banned from London buses this week. The adverts, which read "Not gay! Ex-gay, post-gay and proud. Get over it!", barely register as offensive in comparison with the real horror of gay-to-straight therapy.

The event is largely for the 30 Christian mentors and church members in attendance, though I'm posing as a young man having difficulty dealing with his sexuality. On entering, Dr Lisa Nolland riffles through my bag. A member of the dubiously named Anglican Mainstream, she became a footnote in the history of internet jest in 2010 when she claimed that Cheltenham's innocuous Greenbelt Festival was responsible for the "gayification" of society.

Mr Pickup himself used to be gay, or, rather, he used to be homosexual. He differentiates the two, and believes that to be gay is to act on homosexual impulses and adopt the lifestyle of a gay man. His central idea is that: "Gay identity is a construct, but it's very real to the person who has constructed it, in that it's based on unfulfilled needs and trauma." Despite his own experiences, his view of what being gay entails is narrow. Gays are promiscuous ("They don't get a lot of love, they get a lot of sex"), and are invariably the victims either of sexual abuse or of profound psychological damage. He admits that clients often leave his care just as depressed as when they arrive, but there's no question that he believes wholeheartedly in what he preaches. If it turns out that there is a gay gene, then he would acknowledge the harm he had caused.

Perhaps these opinions aren't extreme enough for those in attendance. "I can see these people are broken, but I feel such anger towards them. I just want to hit them," one man bursts out. Mr Pickup calms him: "Gay militants are grooming our children on an industrial scale. If a gay teacher tries to impose their beliefs on your child, do not hit them. Take out your sword of righteous anger, your biblical sword. Take out your sword of compassion, because it cuts clean." I have to stifle a laugh.

Jacob Wilson, 25, and Peterson Toscano, 47, are unable to joke about their experiences of reparative therapy. They label themselves as "survivors", and believe that those who tried to convert them, in the US and the UK, are monsters. The sessions they had in therapy have left them feeling guilty and ashamed long after their final acceptance of their sexuality.

Mr Toscano began seeking treatment at 18. He claimed that most of his early therapy sessions in New York suggested: "If you memorise enough Bible passages and fill yourself up with Jesus you'll dislodge the gay." He wanted something more "hardline", however, and started seeking exorcisms. One of the treatments also took place in New York, where women chanted around him, urging him to throw up into a bucket: "They were thrilled to see the vomit because it was evidence the demons were coming out," he said. "I was coughing so hard and so long that there was blood, which they took as a sign the demons were being pulled up from the roots, and they rejoiced in celebration."

Still not "cured", he came to the UK. At a church in Kidderminster in 1996, he underwent his third exorcism. "It was very British," he says. "It was gentler than the other two, less crazy. There was a game reserve nearby [West Midlands Safari Park] and they joked that the demons inside me would leave my body and possess the baboons as Jesus cast the demons into a herd of pigs."

A later spell at Love in Action, a conversion camp in Memphis, convinced Mr Toscano that he was gay and left him more at peace with himself, but for Jacob Wilson, memories of his treatment at the same camp have never left him. Mr Wilson, from Salem, a town with a population of just 5,000 in Missouri, was sent to the camp in 2005 at the age of 19. He was outed by his church for having an affair with his pastor.

At the Love in Action facility, Wilson was punished because, as he stretched, he showed his belly button, and this apparently caused another boy temptation. He was banned from wearing Abercrombie and Fitch or Calvin Klein underwear because they were "too gay" and "drew too much attention to the genitals". In his eight hours of daily therapy, "they made it seem like the happiness of our families was reliant on us getting better, the whole weight of the world was on us... For a 19-year-old it was very traumatic." Wilson moved away from home and now lives elsewhere in Iowa, and accepts that he's gay, but he hasn't set foot in a church for more than three years.

Back at the London conference, Mr Pickup is entertaining the conference with choice maxims such as "letting go of shame feels better than sex with a guy" and "the gays want all men to hold hands". Last night, Dr Mike Davidson, co-director of Core Issues, insisted that the quotes I recorded from the session did not represent the spirit of Mr Pickup's talk: "I don't think that was the reality of what David was communicating to the audience," he said.

I left the meeting feeling deeply depressed. But my short taste of gay-to-straight-conversion was nothing compared with that of Mr Toscano and Mr Wilson. They truly are "survivors".