Prime Minister Malcolm Turnbull says Labor's opposition to the proposed plebiscite on same-sex marriage over mental health concerns is "ridiculous".

Labor has vowed to vote against the legislation to enable the public vote, citing concerns from gay and lesbian community advocates that the plebiscite could stoke homophobia and damage the mental health of young people coming to terms with their sexuality.

Mr Turnbull told the Lower House he had discussed the issue with prominent mental health advocate Professor Patrick McGorry, who highlighted "some small elements" of the debate which could be harmful.

But he told Question Time the plebiscite should proceed anyway.

"Are we going to say that we may not have a public debate on a topic when it is alleged that there will be a minority, some small groups, that will act intemperately?" he said.

"Labor's point is to say that because they believe a small number of people will participate in the debate in an intemperate and harsh way, perhaps in a discourteous and cruel way, then everybody else in Australia, the other 99 per cent of Australians, should not be able to have a say.

"That's ridiculous. It is completely ridiculous.

"If that proposition were accepted, how are we going to go with a referendum on constitutional recognition?

"Are we not going to have that because there are some people that will make unkind, racist remarks?"

Mr Turnbull's comments came in response to a question from Labor MP Susan Lamb, who used her first speech this week to make an emotional case for same-sex marriage.

Opposition Leader Bill Shorten moved an amendment to the Government's plebiscite bill, which would see the nationwide poll replaced with a free vote in Parliament.

Mr Shorten told the chamber that no one could guarantee the plebiscite campaigns would be civil.

He said Labor could not be part of legislation that could see the "demonisation" of the LGBTIQ community.

"I have been criticised by some, including members in this chamber, for drawing the link between this plebiscite and concerns about the mental health of LGBTIQ Australians," he said.

"I make no apologies for bringing attention to this most serious issue."