The ideological parent of India's ruling Hindu nationalist Bharatiya Janata Party (BJP) has backed the government position against recognising same-sex marriage, months after raising hopes with supportive comments on gay rights.
Prime Minister Narendra Modi's BJP government has opposed recognising same-sex marriage and urged the Supreme Court to reject challenges to the current legal framework lodged by LGBT couples.
Final arguments in the case are due to be heard by a five-judge bench starting April 18.
"Marriage can only take place between persons of opposite genders, we agree with the government's stance on same-sex marriage," the Press Trust of India news agency quoted Dattatreya Hosabale, a top official of the Rashtriya Swayamsevak Sangh (RSS), as saying.
Hosabale's office confirmed his comments to Reuters.
RSS chief Mohan Bhagwat had said in January that the lesbian, gay, bisexual and transgender (LGBT) community "should have their own private and social space as they are humans and have the right to live as others".
Although Bhagwat had not referred specifically to same-sex marriage, his comments could force the government to reassess its opposition, a junior minister in the federal government and a senior BJP leader had said at the time.
The RSS, established in 1925, is a powerful Hindu group estimated to have millions of active members across India and overseas. The organisation played a major role in Modi's rise to power.
India decriminalised homosexuality when it scrapped a colonial-era ban on gay sex in 2018, but it remains a taboo topic in this socially conservative country of 1.4 billion.
The Modi government has argued that any change to the legal structure of marriage should be the domain of the elected parliament, not the court.
The Supreme Court started hearing petitions to recognise same-sex marriages after four gay couples stated that without legal recognition, they could not have access to rights such as those linked to medical consent, pensions, adoption or even club memberships.