More than 20 countries have now legalized same-sex marriage. Ireland's recent constitutional referendum vote in favor makes Australia look particularly backward in comparison with most other developed, English-speaking countries. Canada, New Zealand and the United Kingdom -- though excluding Northern Ireland -- have also introduced same-sex marriage.
The majority of American states now have same-sex marriage. The U.S. Supreme Court is expected to hand down a decision this year that may confirm whether same-sex marriage is constitutionally protected.
So, as Opposition Leader Bill Shorten introduces an amendment to the Marriage Act to legalize same-sex marriage, why has Australia lagged so far behind?
The question seems all the more perplexing because the Australian federal parliament has the power to introduce same-sex marriage. That means implementing same-sex marriage is a relatively simple matter in Australia -- unlike in the U.S., for example, where there were separate struggles to introduce it in multiple states.
Similarly, there is no marriage clause in the Australian Constitution that requires changing -- unlike in Ireland.
The answer to why Australia hasn't already introduced same-sex marriage partly lies in the way in which the issue has been exploited in Australian electoral politics.
There was no definition of marriage in Australian federal legislation that specified which sexes could marry. But in 2004, then-prime minister John Howard introduced legislation banning same-sex marriage.
Undoubtedly, Howard personally opposed same-sex marriage. However, he also hoped that he could use "values" issues in the 2004 election to wedge socially conservative religious voters away from Labor.
Howard implied that Labor had betrayed its suburban, working-class heartland to support the politically correct views of inner-city elites. This was a conservative framing that conveniently overlooked many gays and lesbians also being workers.
Despite some internal dissent, the power of right-wing factions within the ALP won out. Labor voted with the Howard government to ban same-sex marriage. It feared losing key electorates in areas such as western Sydney.
For years, it was left to minor parties such as the Democrats and the Greens to make the public case for same-sex marriage. Unlike the U.S., where party discipline is less tight, the requirement to toe the party line forced prominent Labor supporters of same-sex marriage to publicly state the ALP's official position.
The Howard-era legislation banning same-sex marriage was also harder to challenge because Australia does not have a bill or charter of rights. The Canadian Charter of Rights and Freedoms played a significant role in bringing in same-sex marriage.
Similarly, in the U.S., gay and lesbian activists used legal challenges to laws prohibiting marriage at state level. Such rights litigation was not an option in Australia. Rather, gay and lesbian activists had to rely on trying to change major party politicians' views.
This was not easy. More moderate conservative leaders in other countries, such as David Cameron in the UK and John Key in New Zealand, eventually supported same-sex marriage. Meanwhile, Howard -- and later Tony Abbott -- used socially conservative, religious "values" arguments to try to wedge off Labor votes.
In the 2007 election, then-opposition leader Kevin Rudd supported the legal recognition of same-sex relationships as being equivalent to heterosexual, de facto partnerships. But he also used his opposition to same-sex marriage to reassure socially conservative, religious voters. Julia Gillard did the same.
It wasn't until 2011 that the long struggle within the ALP finally resulted in a change to the party platform to support same-sex marriage.
In 2013 -- prior to his return to the prime ministership -- Rudd shifted too. He announced that he now supported same-sex marriage on the grounds that it was legitimate for the state and the church to have different policies on marriage in a secular society.
Had Labor's support for same-sex marriage been a binding vote -- as was the case with opposing same-sex marriage or with the introduction of the 2008 reforms legally recognizing same-sex relationships -- the Labor government could have been close to having the numbers to introduce same-sex marriage during the Gillard or second Rudd government. However, Labor MPs were granted a conscience vote.
That conscience vote indicates that the separation between religion and the state is not yet fully accepted in Australian politics. Labor policy had left the question of whom religious organizations should marry, which is clearly an issue of religious conscience, for religious bodies themselves to decide according to their (varied) beliefs.
Labor MPs were effectively being granted a religious conscience vote on the issue of whom the state should be able to marry in a country where currently 72.5% of couples who marry choose to have a civil rather than religious ceremony. State-sanctioned same-sex marriage was still being framed partly as a personal morality issue rather than predominantly as an equality issue.
By introducing a bill before this year's party conference, Shorten appears to have headed off a push for a binding vote for Labor MPs.
The residual religious framing is one of the reasons same-sex marriage has not yet been introduced in Australia. It has proved much easier (and quicker) to introduce in countries that have predominantly constructed same-sex marriage as an equality issue, such as Canada.
Ireland's referendum vote is so important because it demonstrates that, even in a country that is traditionally staunchly Catholic, the majority of voters understand that religious definitions of marriage should be confined to religious marriage ceremonies and not be imposed on the state's definition of marriage.
Most Labor politicians in Australia now take that position too. This includes some MPs who were previously concerned about the impact of same-sex marriage on their seats, such as Chris Bowen, Tony Burke and Ed Husic -- all of whom occupy western Sydney electorates.
Meanwhile, many Liberals are urging that they should be granted a conscience vote on legalizing same-sex marriage. They are likely to be given one. Previously, Liberal MPs haven't had a choice over whether same-sex marriage is constructed as an issue of equal rights for loving adults -- it has only been constructed as a Howard-era values/morality issue.
The Liberals now risk having same-sex marriage used against them electorally just as they once tried to use it against Labor. Shorten suggested that Abbott is: "... stopping Australia becoming a modern nation."
Labor's moves prompted Abbott to argue that any decision to legalize same-sex marriage should be "owned by the parliament, and not by any particular party".
Shorten subsequently suggested that a Liberal could co-sponsor the bill, rather than his deputy Tanya Plibersek. Liberal MP Warren Entsch has indicated his willingness to be involved in future negotiations, but not to co-sign Shorten's current bill.
Irish Taoiseach (Prime Minister) Enda Kenny argued that his country's equality vote: "... disclosed who we are -- a generous, compassionate, bold and joyful people."
Kenny's words raise questions about what message Australia's intransigence on same-sex marriage up to now is sending internationally -- as well as to its own gay and lesbian citizens.
Carol Johnson is a professor of politics at the University of Adelaide. She's also the author of The Labour Legacy: Curtin, Chifley, Whitlam, Hawke and Governing Change: From Keating to Howard. The opinions expressed in this commentary are solely those of the author. CNN is showcasing the work of The Conversation, a collaboration between journalists and academics to provide news analysis and commentary. The content is produced solely by The Conversation.