Only a man and a woman can constitute a marriage, says the Coalition.

Opposition Leader Tony Abbott said the shadow cabinet resolved Monday to maintain the Coalition's current position on the same-sex marriage push, ahead of a likely Labor conscience vote on the issue.

Abbott told reporters the Coalition's position is a matter of maintaining their commitments to the voters.

"The ultimate obligation of politicians is to maintain their commitments to the electorate," he told reporters in Canberra. "It would be a pretty conscienceless opposition if we were to say one thing before the election and say a different thing after the election."

Abbot said the reasons for keeping the Marriage Act unchanged are in the interest of the public.

"It's very important that we respect the importance of families ... and the rights of children to at least start life with a mother and a father," he said.

Abbott did not say if offering a conscience vote had been canvassed at the shadow cabinet meeting. It is not clear yet whether party members would be allowed to speak their minds when the issue comes before the parliament, The Australian reported.

Labor MPs were granted a conscience vote at their recent national conference; Prime Minister Julia Gillard has earlier said the Coalition MPs should be allowed the same.