As Ben Kelley’s answer reflects, but not enough answers have acknowledged, dragging one’s feet about gay marriage has become a bipartisan thing.
Gay marriage has become a flashpoint for the current culture war in Australia; the ex-PM and leader of the conservative faction of the Liberals, Tony Abbott, announced that if you’re sick of political correctness, you should vote against.
The inaction is partly because culture war issues are much more prominent in Australian politics than it was a decade ago. It’s something that conservative commentators, such as Andrew Bolt and Abbot’s former chief of staff Peta Credlin, use as a cudgel against current PM Malcolm Turnbull, who is known to be personally pro gay marriage. “Aussie families don’t care about gay marriage! They care about their power bills!” (Because, presumably, gay couples aren’t real Aussie families to them.)
But more importantly, it is because both parties have been much more riven by internal conflict and factionalism than they were (as witnessed by the revolving door of PMs in the past several years); and progressives in the parties can’t afford to antagonise the conservatives in the parties. The issue is certainly a proxy war between moderates and conservatives among the Liberals; contrast Abbott’s stance with Christopher Pyne’s leaked gloating that the moderates were on the ascendancy within the party, and marriage equality was a matter of time.
Labor has no right to be smug about this now, because Labor was just as captive to its own conservatives when it had the chance to legalise gay marriage. Because of how Labor works, the most prominent opponent was not a member of parliament: it was union head Joe de Bruyn, whose opposition is founded on Catholicism.
The late Gough Whitlam, sainted progressive PM of Labor, was always ready with a quip. Here’s Joe de Bruyn – Wikipedia on de Bruyn on gay marriage:
The SDA [de Bruyn’s union] is associated with the Labor Right, Labor Unity or Centre-Unity grouping or faction of the trade union movement and the Australian Labor Party. It also has a long-established reputation as a supporter of conservative Catholic parliamentarians. De Bruyn, himself a Catholic, is a leading figure in the right wing faction of the trade union movement and the Australian Labor Party. De Bruyn has come under scrutiny for voicing his socially conservative views while being secretary of a trade union and holding a position on the National Executive of Labor, a centre-left political party. He has repeatedly voiced opposition to abortion, and to legalising same sex marriage.
In response to a 2014 poll with 72 percent support for same-sex marriage, de Bruyn dismissed the figures but refused to poll his members on the issue. He says he “knows they agree with him absolutely. When we talk to our members about out these things they agree with us”.
At a quarterly SDA members meeting in February 2011, de Bruyn moved a resolution against gay marriage, without giving any members a chance to speak or vote on the issue. This led to the first instance of members of the SDA speaking out and challenging de Bruyn on his stance on gay marriage. Speaking at an AWU event in 2003, former Labor Prime Minister Gough Whitlam quipped that “Joe de Bruyn is a Dutchman who hates dykes.”
Labor is pro gay marriage now. But that’s easier in opposition than government.