India's top court on Monday referred petitions seeking legal recognition of same-sex marriages to a larger, constitutional bench next month, a day after the government said it opposed the unions.

On Sunday, the conservative government of Hindu nationalist Prime Minister Narendra Modi told the Supreme Court that it was against same-sex marriage and that any change was up to parliament, not the courts.

Its submission said that "any interference... would cause complete havoc with the delicate balance of personal laws in the country and in accepted societal values".

"Living together as partners and having sexual relationship by same sex individuals... is not comparable with the Indian family unit concept of a husband, a wife and children", it added.

The Supreme Court on Monday referred the case to a five-judge special constitutional bench.

It will determine whether recognition of same-sex marriage is valid under the constitution.

The next hearing is expected to take place on April 18, with the proceedings live-streamed, the court said.

"We remain very hopeful... We are very pleased that this matter has gone to a constitutional bench as we consider it to be an issue of fundamental and constitutional rights," said Niharika Karanjawala, a lawyer representing one of the petitioners.

In 2014, transgender people were given official recognition as a "third gender" and three years later India's highest court recognised sexual orientation as protected under a fundamental right to privacy.

A landmark ruling in 2018 struck down a colonial-era law that banned gay sex, and last year the court ruled that unmarried partners or same-sex couples were entitled to welfare benefits.

LGBTQ rights in India have expanded in recent years but remain a sensitive subject in largely conservative and deeply religious Indian society.

"What happened with the 2018 judgement is that homosexuality has been decriminalised. Which means the community... is no longer seen in the same bracket as criminals, murderers, thieves and all of that," said gay rights activist Anjali Gopalan.

"However, no other rights have been granted to the community, for example, rights that we as citizens of this country take for granted and the most obvious one is the right to marry."

Abhay Dang, one of the petitioners, said this year he and his partner were "just strangers" in the eyes of the law, despite having a wedding celebration in 2021.

"Whatever basket of rights marriage provides, which heterosexual couples completely take for granted, for us same-sex couples, we did not have those rights," he said. (AFP)