Coalition backbencher Andrew Hastie warns religious liberty could be limited if same-sex marriage is allowed.

Mr Hastie says marriage is the people's institution and people should have their say on what it means in a plebiscite.

"Changes do not come without consequences," Mr Hastie writes in The Australian on Tuesday.

"Redefining marriage will potentially limit the nature and scope of religious liberty in this country."

The government was interested in marriage because it produced children, he said, with broken marriages leading to increases in welfare and bureaucracy.

Mr Hastie disagrees with coalition colleague Dean Smith, who argues a plebiscite on the matter will undermine the principle of parliamentary sovereignty.

"We are not seeking to redefine parliament. We are seeking to redefine marriage," Mr Hastie writes.

"Where Smith invests authority in parliamentary sovereignty, I choose to invest it in the people."

The existing Marriage Act contains provisions to protect religious ministers and celebrants who want the right to refuse to solemnise a marriage.

Meanwhile, conservative Liberal backbencher Cory Bernardi says marriage equality campaigners are alienating mainstream Australia and doing the biggest disservice to their cause.

Labor Leader Bill Shorten's argument that a plebiscite could drive some gay Australians to suicide was "vile".

"Suicides in Ireland actually went down," he told Sky News in New York of the Irish popular vote on gay marriage last year.

"This is scaremongering writ large - they think they might lose and that doesn't fit in with their agenda."

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