They will do this because, as conservatives, they believe in marriage.

They believe marriage will strengthen same-sex relationships and bring families closer together, while marriage itself will benefit from same-sex couples who want to uphold its traditional values.

They will also vote in favour of protections for religious ministers in any amendment to the federal Marriage Act, following a sensible amendment to the motion from Nationals MP Trevor Khan.

The reason they are free to act on their conservative principles is Premier Barry O’Farrell is sticking to Coalition tradition and granting a free vote.

This means today’s vote will make history for being the first time Coalition MPs will vote for same-sex marriage.

This will send a strong message to Canberra that conservative politicians support equality because of their conservative principles and should be able to act on these principles when voting. Unlike Premier O’Farrell, Opposition Leader Tony Abbott is still denying a free vote at the federal level. This is despite the very clear message from recent opinion polls that the Coalition will suffer electorally if Tony Abbott keeps refusing Coalition MPs and senators the right to vote according to their conscience.

Abbott’s stance is the first time ever that the federal Coalition will be denied a free vote when the ALP has one. His rationale is that he made a promise at the last election not to support reform. But no promise was made that he would break Coalition tradition and deny a free vote, and polling proves voters know this.

Last weekend a Galaxy poll showed 77 per cent of Coalition voters want the Coalition to have a conscience vote on the issue. Another recent Galaxy poll showed 73 per cent of voters believe the Coalition’s tradition of allowing a conscience vote on such issues is what should guide Coalition policy, not Abbott’s personal views or undertakings.

The same poll showed 61 per cent of Australians want same-sex marriage in this term of government, not sometime down the track. Is this feeling strong enough to count at the ballot box? Bob Katter thinks so. During a discussion with Kevin Rudd at the Sydney Writers’ Festival, Katter conceded his Australia Party lost 7 per cent of its first-preference votes because of the anti-gay marriage ad campaign it ran.

Growing support for marriage equality also suggests Abbott’s stance will be a liability for the Coalition. The overwhelming response to recent parliamentary inquiries into marriage equality showed Australians are more highly motivated than ever to support the issue.

A senate inquiry received an unprecedented 44,000 submissions in favour of marriage equality and a house of representatives inquiry received 177,000 positive responses. This makes legislation to allow same-sex marriages the most popular in our parliament’s history.

What this means for Abbott is clear – his hardline stance will guarantee the Greens will have the balance of power in the senate after the next election.

Thanks to Julia Gillard’s continued and clear opposition to equality, the protest vote against Abbott will go straight to Christine Milne.

O’Farrell understands that allowing some MPs to vote for marriage equality proves that the Coalition is a “broad church”.

Abbott doesn’t get this, but he will at the next election if he doesn’t change his mind. His stubbornness will make life very difficult for inner-city Liberals such as Kelly O’Dwyer, Malcolm Turnbull and Teresa Gambaro, whose polling shows support for marriage equality running as high as 75 per cent.

Unless Abbott allows a free vote the issue will plague these and other Liberals throughout the next election. It will be raised at every public meeting they attend. It will distract them from the issues they want to focus on. It will be used to criticise the Coalition for being out of step with the strong support for marriage equality among young Australians.

Today, the NSW parliament is set to send a strong message of hope to gay and lesbian Australians. By denying the federal Coalition a free vote, Tony Abbott is sending the Greens a message of hope that they will have the balance of power after the next election.

Alex Greenwich is convenor of Australian Marriage Equality