Sexual orientation cannot be wilfully changed and is a product of genetic and environmental factors, said lawyers arguing for the repeal of Section 377A of the Penal Code.

They argued that the law, which criminalises acts of "gross indecency" between men, violates Article 9 of the Constitution guaranteeing the right to life and personal liberty, and Article 12 guaranteeing equal protection before the law.

The legal team, consisting of Mr Eugene Thuraisingam, Mr Suang Wijaya and Mr Johannes Hadi of Eugene Thuraisingam LLP, represented disc jockey Johnson Ong Ming in the High Court yesterday in the second of three cases to be brought against Section 377A this month.

The Attorney-General's Chambers (AGC) has been listed as the respondent in all of the cases.

Mr Ong's lawyers presented expert evidence from six medical professionals to back up their claims, including three called by Mr Ong and three called by the AGC.

Those called by Mr Ong were British psychiatrist Dinesh Bhugra, a professor of mental health and diversity at the Institute of Psychiatry at King's College London; Dr Jacob Rajesh, a senior consultant psychiatrist at the Promises Clinic in Novena Medical Centre; and American public health and epidemiology professor Chris Beyrer of the Johns Hopkins Bloomberg School of Public Health.

Those called by the AGC were Dr Cai Yiming, an emeritus consultant in the Department of Developmental Psychiatry at the Institute of Mental Health; retired geneticist John Tay Sin Hock, the former head of the Human Genetics division at the National University of Singapore; and Dr Derrick Heng Mok Kwee, group director of the Health Ministry's Public Health Group.

The experts on both sides largely agree that sexual orientation cannot be wilfully changed and that one's genes and non-social environmental factors such as exposure to different levels of hormones in the womb are contributors to one's sexual orientation, the lawyers argued.

There is also no credible scientific evidence that "conversion therapy" aimed at changing sexual orientation is safe or effective, they added.

But the experts differed on whether choice and social factors like culture play a role as well.

Dr Cai said there is "very little we can scientifically conclude about whether there is choice in sexual orientation". Dr Tay said that genetics may play some part in determining sexual orientation but is not the sole cause of it, suggesting that culture plays a role as well.

Mr Ong's lawyers contended that the scientific literature Dr Cai cited contradicted his conclusion. They also argued that Dr Tay did not cite any evidence to support his claim that cultural factors play such a role.

"It is absurd, irrational and discriminatory to criminalise a person on the basis of his natural, unchangeable identity and for non-harmful private acts," the team said in a statement to the media yesterday.

The lawyers noted that their case differs from a previous case brought against Section 377A in 2010 by Mr Tan Eng Hong, whose lawyer had argued that a person's sexual orientation is biologically determined. Mr Tan had provided the court with statements from medical and scientific bodies which were not formally entered as evidence, they said.

"For the first time, there is expert evidence before the courts on the nature of sexual orientation," the lawyers said in their statement.

The first of the three recent cases, brought by Mr Bryan Choong Chee Hong, 42, the former executive director of lesbian, gay, bisexual and transgender (LGBT) non-profit Oogachaga, was heard last week.

The third case, brought by LGBT activist and retired doctor Tan Seng Kee, was also heard yesterday.

Mr M. Ravi of Carson Law Chambers, who represented Dr Tan, argued that other laws make it legally obligatory for anyone to report those who violate Section 377A, including gay men themselves, their friends or family members and their medical care providers.

For example, Section 424 of the Criminal Procedure Code (CPC) states that every person aware of the commission of - or intention to commit - any arrestable offence punishable under Chapter XVI of the Penal Code, among others, shall report that commission or intention to the police. Section 377A falls under Chapter XVI of the Penal Code.

Mr Ravi argued that Parliament's stance that Section 377A will not be proactively enforced "interferes with Article 9(1) of the Constitution, as it leads to an inconsistent and arbitrary application of criminal procedure as well as being incongruous with the mandatory obligation" under Section 424 of the CPC.

The three cases were heard by Justice See Kee Oon in chambers and were not open to the public.

The AGC began its submissions yesterday, which are expected to conclude at the next hearing tomorrow.