The votes took place under conscience vote provisions for Labor while the opposition voted in unison against the bill. No opposition member crossed the floor or abstained. Of the major figures in the Parliament Julia Gillard, Tony Abbott, Kevin Rudd and Malcolm Turnbull (the latter under protest) all voted against the proposal while Christine Milne led her whole party in voting Yes. There is general agreement that the votes would have been closer if the opposition parties had allowed a conscience vote but that the bill would still have failed to pass. Labor MPs favoured same-sex marriage by 54 votes to 33.

Is that the end of the matter? Will the issue fade away or is ultimate success for the same-sex marriage cause inevitable? The political commentator Paul Kelly discussed this question in The Australian last weekend and forecasts what he calls a lengthy cultural conflict. In doing so he makes several comparisons with the republic question. But in his discussion he neglects important differences between the politics of the two questions.

The major characteristic of the same-sex marriage debate in Australian politics is that it is an international issue. It is this characteristic that will probably ensure its political longevity. There are many more important political arenas, including Britain and the United States, than Australia for this debate. If same-sex marriage continues to gain momentum around the world then the Australian debate will not go away. If the majority of the world adopts same-sex marriage then the pressure on Australia to do so will eventually be great. But should international interest fade away then it probably will in Australia too.

Whatever the future of republic-monarchy debates in Australia they are not of the same international character and therefore are quite different from same-sex marriage. In the case of the republic there are a number of Commonwealth countries considering the move to a republic from a monarchy, such as Jamaica, but Australia's international comparators are really only Canada and New Zealand. Effectively it is a stand-alone domestic issue.

A second important difference lies in a factor that Kelly does recognise, which is that same-sex marriage is also very much a state and territory issue. The ACT has shown this for some time and several states, including Tasmania, are debating the question. Importantly this means that within Australia there are multiple parliamentary avenues for advocates to pursue the-same sex marriage cause.

This has not been the case with the republic question, which has been centred on national constitutional reform. Certainly the states play a role, as they are constitutional monarchies too, but the state parliaments have never been major arenas for monarchy-republic debates. This difference gives same-sex marriage advocates a big advantage over republicans as they have many more real avenues to take. These avenues can be used for building momentum and ultimately for achieving national-level success. Together with the international character of the issue it means that same-sex marriage can be debated at multiple levels: international, national and state.

Same-sex marriage, unlike the republic, also has an accompanying goal. In this case it is the civil unions question. This may be internally divisive among gay rights activists, just as the debate about republican models divided republican advocates, but it does at least broaden the debate and allow advocates an alternative to consider if same-sex marriage is blocked.

A closer political parallel with same-sex marriage is the euthanasia question, which also has state, national and international elements. However, the international momentum for euthanasia is much weaker and doesn't feed into domestic Australian debates in the same way. Euthanasia is a fringe issue, though it has advocates in each state and territory. It has already had one major national parliamentary debate in 1996-97 on the back of Northern Territory legislation. The Federal Parliament overrode the NT legislation on that occasion.

Finally, at the moment, same-sex marriage is not a constitutional issue like the republic. Constitutional issues have their own politics, most notably, the referendum process. There are some calls for a referendum on same-sex marriage but if one were to occur at the moment it would not be a binding constitutional referendum, under the provisions of s128, but a plebiscite.

Down the track it may become a constitutional issue. This would occur if a state parliament introduced same-sex marriage and the legislation was challenged on constitutional grounds in the High Court, and the challenge upheld. At that stage the politics would enter a new era and success would be harder for its advocates to achieve.

Same-sex marriage is not inevitable, but nor is it unlikely. That is the case both internationally and in Australia. Continuing momentum internationally, in terms of pro-same-sex marriage legislation passed in some other countries, would make its achievement in Australia more likely.

The same would be true if one or more Australian states were able to pass legislation which survived constitutional challenge.

But its advocates should understand, if the republic question is any guide, that political struggles may continue for decades. These are questions of the utmost moment, existing institutions are entrenched, and the opposition to reform is well-organised.

John Warhurst is an emeritus professor of political science at the Australian National University and Deputy Chair of the Australian Republican Movement.

John.Warhurst@anu.edu.au