In Prof Koh's commentary published on Monday - Section 377A: There is a difference between a sin and a crime - he delved into the history of the legislation and scientific studies on homosexuality. 

On the scientific evidence for repealing the law, he wrote: "Scientific research has shown that homosexuality is a normal and natural variation in human sexuality and is not in itself a source of negative psychological effects."

He cited a World Health Organization decision in 1973 to delete homosexuality from the Diagnostic and Statistical Manual of Mental Disorders.

"This decision was endorsed by the general assembly of WHO in 1990 when it agreed to delete homosexuality from its list of mental disorders. In WHO's view, being LGBT (lesbian, gay, bisexual and transgender) is not a disease," he added.

Prof Koh also pointed out that of the 196 states in the United Nations, the majority - 124 states - do not criminalise sodomy. Singapore is among the minority of 72 states which do.

While all the developed, Western countries do not criminalise sodomy, a notable number of Asian territories also fall into this category, including China, Japan, South Korea, Hong Kong, Taiwan, the Philippines and Thailand, he said.

Meanwhile, many of the countries who have a similar law in place were former British colonies.

"Singapore is part of the minority mainly because it inherited from the British a penal code which criminalises sodomy. For a country which embraces science and technology, it is surprising that, on this one aspect, the law has not been updated, in the light of the scientific evidence," he wrote in The Straits Times.

SEPARATION BETWEEN RELIGION AND STATE

In making his argument, he also elaborated on the difference between a sin and a crime: While adultery and fornication are regarded as sinful, they are not "criminal behaviour".

Former attorney-general Walter Woon was "unhappy" with the compromise of "retaining 377A and not enforcing it because it brings the law into disrepute", Prof Koh said.

Addressing the calls of Christian and Muslim religious groups to retain Section 377A, he said: "It is not the business of the state to enforce the dogmas of those religions. In Singapore, there is a separation between religion and the state. Church leaders and Islamic leaders should respect that separation."

He called on Christian and Muslim leaders to refrain from "pressuring the Government to criminalise conduct which they consider sinful".

Laying out court decisions in the US and India which had ruled that laws criminalising sodomy were unconstitutional, he wrote: "In most cases, the antiquated sodomy laws were repealed by the legislatures of the respective states or countries. However, in a few cases, the laws were declared unconstitutional by the courts."

Summarising his arguments, Prof Koh said: "Section 377A is an antiquated law, not supported by science, and should be repealed."

He concluded that the Singapore Court of Appeal should overturn a 2014 decision and declare Section 377A to be unconstitutional.