Australia recognized same sex marriage in 2017. That was 3 years after after the USA did. I'm not sure I would call Australia as being progressive by that measure.
What you have to realize about the US is that the Supreme Court deciding these issues is... controversial. Many view it as undemocratic (although it's worth noting that many of those same people don't feel the same way about the Supreme Court's pro-2nd Amendment interpretations of modern times).
We're still seeing the ripple effects of Roe v. Wade being decided almost 50 years ago.
As others have pointed out, Supreme Court precedent is only that until a later Supreme Court comes along and changes it and that could very well happen with the current court composition. And you need look no further than the Court's refusal to stay the Texas abortion law pending a full judicial review.
So the date this was decided doesn't really give any moral superiority here of the US over Australia for two reasons:
1. Australia's position was decided by a democratic process; and
2. With 62% (IIRC) of people approving of the change, the matter is effectively settled. No one can complain about it. As such you don't see any real conservative Christian blowback like you do in the US as witnessed by the statements of Tony Abbott and others.
Abortion is far less of a political and cultural hot potato in Australia than in the US. While no doubt some of that is due to other cultural differences – for whatever reason, conservative Christianity has never been quite as strong an influence in Australia as in the US – I think a big factor is the difference in how abortion was legalised.
In Australia, there is no constitutional right to abortion. And yet abortion is legal nationwide. Its legality was achieved through a gradual process, involving state legislation, and state court decisions – those state court decisions were not based on constitutional law, rather the state courts simply decided to reinterpret the existing criminal laws against abortion to not apply to medical procedures. The federal courts stayed out of it completely, and constitutional rights never came into it, only statutory law – if they wanted to, the state parliaments could have overturned the state court decisions effectively legalising abortion, but they chose not to.
The fact that the process was much more gradual, and had greater democratic legitimacy, has helped make it less of a controversy. And since it happened through a series of state laws and state court decisions, opponents of legal abortion don't have one single highly visible event to focus all their opposition upon, like Roe v Wade is in the US. Almost every American has heard of Roe v Wade – even many non-Americans have – by contrast, few Australians know about the Menhennitt ruling (which de facto legalised abortion in the state of Victoria in 1969) or the Levine ruling (which did the same in New South Wales in 1971). American opponents of legal abortion focus most of their energy on trying to get justices appointed to the Supreme Court who will overturn Roe v Wade – the main purpose of this latest Texas law is to get a case before the Supreme Court which will give those justices an opportunity to do exactly that – its Australian opponents don't have any national focus, which in practice makes them a lot weaker.
It is totally consistent to support legal abortion, but to also think that Roe v Wade was a strategic mistake.
I wouldn't call the US morally superior here anyways. In fact, i think anyone after the US is really late to the party. The first US law was MA legalizing gay marriage in 2004. It took 11 years of fighting after to get to supreme court.
The recent ruling on the Texas heartbeat law might make a dent in that, although I doubt it.
Changes made by pure fiat don't last. Gay marriage in the US could absolutely turn out to have been temporary for just the reasons you explained.
The US was only the 17th country in the world to allow same-sex marriage nationwide; Australia was the 23rd. Still only 28 out of 193 UN member states have it fully nationwide – less than 15%. Only a 3 year gap, when any country having legal same-sex marriage is just over 20 years old. If we look at this in a global and historical context, both countries are very progressive on this particular issue. To argue that the US is in some significant way "more progressive" than Australia just because it got there a few years earlier, you have to be looking at things from a rather narrow perspective.
"Fully nationwide" above is only counting the main national territory of the country, not overseas territories. Even for the US, same-sex marriage is not legal in the territory of American Samoa – the territorial government of American Samoa claims that Obergefell v Hodges doesn't apply to them due to some legal technicalities, and it appears nobody has yet challenged that claim in court. Likewise, Netherlands, New Zealand, and UK all have legal same-sex marriage throughout their main national territory, but not in some of their overseas territories.
So if you want to be hyper-pedantic about it, the US still doesn't have legal same-sex marriage (throughout all of its territory), Australia does.
The USA recognized same sex marriage in 2015, but we did so via a Supreme Court ruling. The Court's composition has changed since then, and the new Court has signaled a willingness to overturn settled precedents.
While our current detent may persist, I would imagine that an action taken by the Australian Parliament would tend to be more durable.
I'm curious to understand why you'd think a law passed by elected lawmakers is more durable than a verdict by judges who have their position for life? Obacare was surely very controversial, but despite the best efforts by the conservative and an ideologically hostile Supreme Court, it remains in place after ten years.
Those efforts were actually pretty half-hearted. They talked a big game, but they don't actually want to abolish it. (Reminds me of how the UK doesn't really want to face up to a no-deal Brexit, despite lots of brave talk. Or how no government in the UK has so far re-nationalised the railroads, despite re-nationalisation perennially polling high with the general public.)
Legally, Parliament could repeal it tomorrow and ban same-sex marriage again. Politically, that would be impossible without another plebiscite, and there is no reason to believe a new plebiscite would deliver a different outcome; it would almost certainly return the same result, and likely by a bigger margin.
No conservative politician in Australia wants to talk about this. I'm sure some of them are still personally opposed to same-sex marriage, but they all realise trying to repeal it is hopeless, and so they'd rather talk about things that they have some hope of achieving.
The realignment of the Court, plus this Court's willingness to overturn settled law, together signal that a new law prohibiting same-sex marriage might be allowed to stand if brought before today's Court, returning the US to a country where same-sex marriage is not universally legal.
So I would regard our current regime around same-sex marriage, civil rights, etc. to be subject to not being directly challenged at the Supreme Court.