ONE of Australia's leading psychiatrists has joined forces with 150 doctors lobbying the Federal Government to ban same-sex marriage.

Professor Kuravilla George, who is Victoria's deputy chief psychiatrist and the State Government's equal opportunities champion, claims that gay marriage poses a health risk to society.

In a letter to the Senate's inquiry into marriage equality, the group of doctors wrote that it was "important for the future health of our nation" to retain the definition of marriage as being between a man and woman.

"We submit the evidence is clear that children who grow up in a family with a mother and father do better in all parameters than children without," they wrote.

News of the letter, revealed in today's Sunday Herald Sun, follows Attorney-General Nicola Roxon's announcement yesterday that she would vote for gay marriage when the chance came later this year.

Ms Roxon was speaking as thousands of Australians held protest marches across the country calling on Ms Gillard to change her stance after US President Barack Obama said that same sex couples should be able to wed.

Australia passed an amendment to its laws in 2004 explicitly defining marriage as between a man and woman, but there are several bills before the parliament calling for the right to be extended to same-sex couples.

Read the full story on the doctors' letter here (subscribers only).