A Hong Kong judge has ruled on Wednesday that laws prohibiting gay men younger than 21 from having sex are unconstitutional.

Judge Michael Hartmann ruled that the anti-gay laws "discriminate on the basis of sexual orientation" and "are demeaning of gay men who are, through the legislation, stereotyped as deviant."

Although homosexuality was decriminalised in Hong Kong in 1991, gay men who engage in consensual sodomy when either is under 21 face life imprisonment.

The laws, which have been on the books for 14 years, also prohibited "gross indecency" or sexual intimacy between men if one or both are under 21. But heterosexual and lesbian couples who are 16 or older can legally have such relations.

The case was brought by William Roy Leung, a 20-year-old gay man, who challenged the measures - including one that demanded a life sentence for gay sex when one or both men are younger than 21.

In the ruling, Hartmann also said that the laws are a "grave and arbitrary interference with the right of gay men to self-autonomy in the most intimate aspects of their private lives."

Leung said his legal victory means that "I can finally have a loving relationship without being scared of (being) thrown into jail for life imprisonment."

Gay rights group Civil Rights for Sexual Diversities (CRSD) and Sections 118 Concern Group declared the ruling "a historical moment for the Hong Kong gay community."

In a joint statement, the groups hope for the ruling to "set a precedent to uphold sexual orientation as one of the protected grounds against discrimination, provided in Hong Kong's Bill of Rights and the Basic Law" and for "other discriminatory public policies and practices will be challenged on the same basis."

The Court further ruled the offence of gross decency, which lacks a solid legal definition outlawing intimate behaviour between men, to be unconstitutional. The law would however have to be struck down from statute books by a legislative reform.

CRSD's Roddy Shaw said police have arrested 65 men under gay sex laws in the past five years, and 26 were convicted. Those convicted are barred from working as teachers, social workers, in health care and other caring professions. The groups also call for the criminal records of the men to be eliminated.

The ruling came as Hong Kong debates whether a law prohibiting discrimination against homosexuals is needed. While the government has provided few details about what the anti-discrimination bill would entail, Shaw said he thought the new ruling would help advance the bill.