He began going to regular counselling and prayer sessions, during which he was told he could become straight.

Soon he was struggling to sleep, he lost weight and had trouble focusing on his final year of undergraduate study. Doctors diagnosed him with depression.

"It made me feel guilty all the time. I felt ashamed and blamed myself for being different to other people," Cheung said. "I wanted to commit suicide."

Campaigners say that experiences like Cheung's are not rare in Hong Kong. They are calling for a legal ban on such programmes, which they refer to as "conversion therapy", saying they are discriminatory and harmful.

Those running the programmes - most often conservative Christian groups - reject that label and argue that they are simply providing gay people with a "choice".

Homosexuality has been decriminalised for nearly three decades in Hong Kong, a former British colony that returned to Chinese rule in 1997.

But despite its cosmopolitan facade and a vibrant gay scene, including an annual pride parade, gay people often come under family pressure to marry and have children. The city does not recognise same-sex marriage.

LGBT BRAIN DRAIN

Hong Kwai-wah founded the New Creation Association in 2004, a Christian group which he said is simply providing support to people, including those who wish to be straight, because "sexual orientation is fluid and change is possible".

He denied that his group paractices conversion therapy.

"When a person has same sex attraction, it does not mean that they must have homosexual behaviour or develop same sex relationships," Hong told the Thomson Reuters Foundation.

"They still have a choice to be gay or not to be gay."

Hong, a psychiatrist, said he has seen more than 100 gay people change their sexual orientation.

"Banning homosexuals from changing into heterosexuals out of their own initiation is unethical, and clearly a violation of human rights of homosexuals," said Hong.

Advocates for Hong Kong's lesbian, gay, bisexual and transgender (LGBT) community say that sexual orientation is not a choice, and pretending it is only reinforces social stigma.

"It is hugely damaging for the mental health of LGBT individuals," said Gigi Chao, who came out publicly after her tycoon father offered a $65 million reward in 2012 to any man who could marry his lesbian daughter.

She said that stubbornly conservative attitudes towards homosexuality are driving people to leave the financial hub.

"A lot of my close friends who I grew up with, for example - gays or trans - they have chosen not to come back to Hong Kong," Chao said. "They have chosen not to be part of this homophobic culture."

'A RESPONSIBLE SON'

In some countries, gay people are still forced to undergo archaic and invasive therapies, which can involve psychoanalysis, injections and electric shocks.

Such extreme approaches are not known to be practiced in Hong Kong, according to Nocus Yung from Hong Kong-based pro-LGBT group Queer Theology Academy.

But programmes aimed at changing sexual orientation, often done in religious setting, are held discreetly, she said.

Participants are often told that they can change their sexual orientation through prayer, cold showers and practising abstinence to avoid same-sex relationships, Yung added.

Although conversion therapy has been widely discredited, only Brazil, Ecuador and Malta have nationwide bans on conversion therapy, according to the International Lesbian, Gay, Bisexual, Trans and Intersex Association.

Campaigners argue that stigmatising and pressuring LGBT people is rights violation.

Cheung - now in his 30s and a social worker - remembers vividly the 18-month ordeal he went through after he signed up for the programme run by a church group over a decade ago.

"I wanted to be a normal person, a responsible son," he said, as he showed a pile of reading materials from the "gay cure" counselling sessions he had signed up for.

Sunny, a gay man who attended a similar programme for one year until 2016, said he was often made to think that his sexuality was sinful.

Like others, the 22-year-old was told to pray often and abstain from sex to avoid same-sex relationships.

"Even if we cannot become straight, they told us it is better for us to be single than be a gay," said Sunny, who declined to give his full name, because he has not come out publicly.

Hong Kong's Equal Opportunities Commission (EOC) has called for legislation banning discrimination against LGBT people.

"The EOC believes that nobody should be denied their right to a dignified life because of their sexual orientation or gender identity," the government-backed commission said in an email.

But conversion therapy remains a divisive topic even among Christians in Hong Kong, where many reject it, including Peter Koon, a leader in the Anglican Church.

Christianity is one of the main religions in Hong Kong. There are some 480,000 Protestant Christians and 379,000 Catholics in the city of seven million, according to official figures.

"In terms of acceptance of gay people, we have a common understanding of God's love for everyone," Koon said.