A draft amendment put forward by the Russian president would alter the country’s founding document to state that marriage is a “union of a man and a woman”.
It is one of several proposed changes that would make the constitution more conservative.
Other amendments include specific mentions of Russians’ “belief in God” and an homage to “ancestors who bequeathed to us their ideals”.
The president first proposed updating the constitution in his state-of-the-nation speech of January. He claimed it was necessary to broaden the powers of parliament and bolster democracy, but opponents say the move is part of his efforts to remain in power at the end of his six-year term in 2024.
The Kremlin-controlled parliament quickly endorsed Mr Putin’s initial draft amendments in the first of three required readings last month.
Following proposals from a Kremlin working group that operates parallel to MPs, he tabled 24 pages of additional amendments on Monday ahead of the second reading on 10 March.
They are likely to get final parliamentary approval next week, setting the stage for a nationwide vote on 22 April.
Other amendments proposed for inclusion in the revised constitution define Russians as a “state-forming” ethnic group.
After proposals to outlaw disparaging the Soviet role in the Second World War, Mr Putin added an article pledging to protect “historic truth” and forbid “belittling the people’s heroic protection of the Motherland”.
Another amendment states Russia should never surrender any territory — a change proposed in response to a working group member’s suggestion of measures to prevent any future ruler from giving away Crimea, the region annexed from Ukraine in 2014.
Mr Putin, who has aligned himself with the Russian Orthodox Church and sought to distance his country from liberal Western values, is said to see the constitutional overhaul as an opportunity to enshrine what he sees as his country’s core moral and geopolitical values.
The president vowed last month that Russia would not legalise gay marriage as long as he was in the Kremlin, saying he would not let the traditional notion of a mother and father be subverted by what he called “parent number 1” and “parent number 2”.
Homosexuality in Russia was a criminal offence until 1993 and classed as a mental illness until 1999, and same-sex couples are still banned from adopting children.
Homophobia remains widespread in the country, and western governments and human rights activists have criticised the Russian authorities for their treatment of LGBT+ people.
A law introduced to ban “gay propaganda” in 2013 has been been used to stop pride marches and detain rights activists.
Mr Putin was claimed he is not prejudiced against gay people, but says he finds a Western willingness to embrace homosexuality and gender fluidity out of step with traditional Russian values.
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