The government's same-sex marriage legislation is "absurd" and would enshrine new discriminations for religious groups and marriage celebrants to discriminate against gay couples, say anti-plebiscite campaigners.

Attorney-General George Brandis late on Monday released the draft exposure bill that would be passed in the event of a successful plebiscite. But that prospect appeared doomed on Tuesday as the Labor opposition prepared to formally announce it would vote down the government's plebiscite bills in the Senate and continue to press for a parliamentary vote instead.

The draft same-sex marriage bill would replace the definition of marriage as between "two people" instead of "a man and a woman".

The bill would also strengthen and retain protection for religious ministers refusing to marry same-sex couples, while non-religious celebrants would be allowed to refuse to marry a couple on "conscientious" grounds.