"Conservatives who oppose same-sex marriage and a conscience vote will be sitting pretty. Tanya Plibersek will be the first line in their argument. Wants Labor MPs to have a binding vote on gay marriage: Acting Labor leader Tanya Plibersek. Credit:Steven Siewert "This has put the cause back and she needs to explain herself to same-sex marriage proponents. "There has been a slow and cautious approach to achieving a conscience vote and she has wrecked that." Senator Smith said he suspected Ms Plibersek's position was more about internal Labor politics than advancing the cause of same-sex marriage.

Labor's platform currently supports same-sex marriage but does not make it compulsory for Labor MPs to support it in a parliamentary vote. Opposition Leader Bill Shorten is on the record as a supporter of a conscience vote on the issue. Liberal senator Dean Smith. Credit:Philip Gostelow The fact that Labor has allowed its parliamentarians a conscience vote has been one of the primary arguments of those lobbying for the Liberal Party to overturn its binding opposition to gay marriage. Liberal politicians have traditionally prided themselves as having more freedom to vote according to their conscience than Labor politicians, who risk expulsion from the party for crossing the floor. "I have always been distrustful of the Left on this issue and now my personal fears have been realised," Senator Smith said. A spokesman said Ms Plibersek would not comment on internal Liberal Party issues. Earlier she said Labor should adopt a binding vote on the issue because same-sex marriage is an issue of legal discrimination, not conscience.

"Labor has always been a party that is opposed to discrimination," she said. "It is a clear question. Do we support legal discrimination against one group in this country? Or do we not?" There has been a slow and cautious approach to achieving a conscience vote and she has wrecked that Senator Smith disagreed: "People's attitudes to abortion and euthanasia are influenced by spiritual positions and that fits into the debate on same-sex marriage." Senator Smith said it is "inconceivable" that Labor politicians could face expulsion from the party for voting against same-sex marriage. Senator Smith revealed earlier this year that he had abandoned his longstanding opposition to same-sex marriage.

"It's getting harder and harder to justify opposing same-sex marriage in Australia," he said at the time. "Is this really the business of government in today's day and age? I'd argue no." Liberal Democrat Senator David Leyonhjelm, who has introduced a private member's bill to legalise same-sex marriage, also rejected Ms Plibersek's push for a binding vote. "I think there should be a conscience vote for all politicians," Senator Leyonhjelm said. "Nobody should be forced to vote against their values on this." Last month Assistant Treasurer and rising Liberal Party star Josh Frydenberg came out in support of a conscience vote.

"I think if you look at the history of movements such as this, I think one thing is certain that over time, gay marriage will become part of the Australian way of life," he said.