A High Court challenge could delay the Turnbull government's plans to hold a national ballot on same-sex marriage.
Labor, the Greens and Nick Xenophon Team senators on Wednesday blocked the government's second attempt at passing laws to enable a $170 million compulsory plebiscite.
Finance Minister Mathias Cormann shortly afterwards said the backup plan - a $122 million voluntary postal ballot run by the Australian Bureau of Statistics - would go ahead.
Ballot papers are due to hit mailboxes from September 12 and the final result should be known by November 15.
But independent MP Andrew Wilkie and marriage equality advocates Shelley Argent and Felicity Marlowe have initiated High Court action to head off the ballot.
They have been advised there are constitutional problems with the ABS running the poll and the government paying for it without parliament's approval.
Senator Cormann said the government's advice was there were no constitutional or legal problems with the postal vote, which if passed would lead to a marriage law change being brought to parliament.
Labor frontbencher Penny Wong said the parliament should debate and pass changes to marriage laws now, rather than wait for the results of an "expensive stunt".
Senator Wong, who is in a same-sex relationship, said many in the Liberal-Nationals coalition could not countenance equality.
Senator Wong objected to the Australian Christian Lobby describing the children of same-sex couples as the "stolen generation" and rejected Senator Cormann's statement the plebiscite could be a "unifying" decision for the country.
"You talk about unifying moments? It is not a unifying moment. It is exposing our children to that kind of hatred."
Senator Cormann said it was important for the debate to be conducted with "courtesy and respect".
Labor leader Bill Shorten asked Prime Minister Malcolm Turnbull in parliament why the government was spending $122 million on a poll when the money could be used to help 334,000 pensioners with their power bills.
Shadow attorney-general Mark Dreyfus appeared to give up on fighting the postal ballot, tweeting on Wednesday: "We might not like it, but the marriage equality postal vote is happening - and we have to win it. Enrol now!"
The Australian Electoral Commission has urged all people to check and update their enrolment details if necessary.
Former prime minister Tony Abbott, who opposes same-sex marriage, earlier told reporters Mr Turnbull should be congratulated for allowing the vote to go ahead.
He framed the debate much wider than a question about changing marriage laws.
"If you don't like same-sex marriage vote 'no', if you're worried about religious freedom and freedom of speech vote 'no', if you don't like political correctness vote 'no'," Mr Abbott said.