Leading "no" campaigners, including Turnbull government MPs, say they fear it will become illegal to oppose same-sex marriage in word or even thought, if gay marriage is legalised.

The extraordinary claims, made at the campaign launch for the Coalition for Marriage on Saturday night, went as far as expressing fear that thought crime would be punished by law.

Matthew Canavan, a member of Prime Minister Malcolm Turnbull's cabinet until he resigned over his dual citizenship, told the 1500-strong Sydney audience: "The 'yes' side want to make it illegal to just express a different view about marriage, that is their agenda."

On the sidelines, he told Fairfax Media he feared "a strong push to effectively eradicate the view that marriage should be between a man and a woman, to make it illegal".