Last week’s victory for same-sex marriage at a referendum in Ireland, and the prospect that Australia might shortly follow suit by the grace of Parliament, are the latest signposts in a remarkable shift in public attitudes throughout the Western world over the past few decades.

In the early 1990s, a quarter of Australian respondents to the World Social Values survey reported that they would not care to have homosexuals as neighbours, something barely half as many would own up to two decades later.

A few years before that, the first survey of the Australian National University’s Australian Election Study series, which was conducted at the peak of the AIDS panic in 1987, found only three in 10 Australians were unambiguously of the view that homosexual acts between consenting adults should be legal.

It’s hard to say how such a question would go in today’s environment, since no one bothers to ask it (although trends in the United States may be thought instructive).

According to the most recent result from Essential Research, not even three in 10 will now sign on to the much more modest proposition of opposition to same-sex marriage, for which support was rated at 59%.

The social transformations that underlie such changes in attitudes have been reflected by the Australian Bureau of Statistics’ newly adopted practice of providing census data on the rate of same-sex relationships around the country. This offers a helpful insight into the implications of the “pink” vote as a factor that MPs might care to keep in mind when exercising their consciences in a looming vote on same-sex marriage.

As you might expect, the three electorates with the highest concentration of same-sex couples are those of inner Sydney, each of which is served by one of the heaviest hitters in Australian politics — Tanya Plibersek in Sydney, Anthony Albanese in Grayndler and Malcolm Turnbull in Wentworth.

Other electorates near the top of the table are also in or just outside of the metropolitan city centres, with the electorates of Melbourne, Brisbane, Perth and Adelaide all placed in the top 13.

While these areas are notable for their concentration of Greens support, research by Luke Mansillo of the University of Sydney cautions against a view of the pink vote as a green-left monolith — at least so far as the male component of it is concerned.

After controlling for socio-economic factors, Mansillo’s study found no support for the notion that homosexuality in and of itself helped explain voting patterns among men. But it was quite a different story with the data for lesbian couples, who appeared to drive significant increases in the Labor and Greens vote.

This is consistent with the disparity between the two seats that neighbour the electorate of Sydney — Grayndler to its west, encompassing Marrickville and Leichhardt, and Wentworth to the east, which spans from Darlinghurst to Bondi.

While the two electorates have roughly the same number of same-sex couples overall, in Grayndler the gender balance is exactly even, such that it outpaces Sydney to record the nation’s highest number of female same-sex couples, whereas Wentworth has fully four times as many male same-sex couples as female. And sure enough, the Liberals’ grip on Wentworth has never loosened, while in Grayndler it fights it out for second place with the Greens.

This reflects a broader trend in which lesbian couples are relatively more prominent in the second ring of electorates out from the city centre, such as Batman and Wills in Melbourne and Lilley in Brisbane, while male couples dominate the higher-rent central business districts.

At the opposite end of the spectrum, same-sex couples are thinnest on the ground in the white suburban Bible belt (Aston and Menzies in Melbourne), Vietnamese and/or Arabic community enclaves (McMahon and Fowler in Sydney), and electorates distant from the capital cities (Grey in South Australia, Flynn and Maranoa in Queensland).

One thing that electorates at both ends of the spectrum have in common is that they tend not to be where elections are won and lost. Certainly the Greens are giving both parties something to think about in inner-city Sydney and Melbourne, but the seats that might be thought most resistant to gay marriage are safe for either Labor or the Coalition, depending on their ethnic profile.

It also doesn’t seem that sensitivity to constituents offers much of a guide to how members might line up on same-sex marriage, at least to the extent that Labor’s conscience vote in 2012 offers a guide.

Once the exceptional cases of Tanya Plibersek and Anthony Albanese are removed from the equation, there was hardly difference between the same-sex relationship average in seats held by the 37 members who voted yes (0.67%), and the 24 held by those who voted no (0.60%). Far more instructive was factional alignment, with members and senators from the left breaking 35-8 in favour, while those on the right went 27-17 against.

Of course, the issue for Labor MPs on that occasion was purely symbolic, as the bill was doomed to defeat in any case — either by Labor allowing a conscience vote or the Liberal Party not allowing one, depending on how you prefer to read it.

Should things play out differently this time, history may record the irony that that the decisive role was played by a shift in policy from an arch-conservative prime minister.

Indeed, to the limited extent that the episode will have any real bearing on the broader electoral contest, it may well be in boosting the cause of Tony Abbott, who will be able to show the consistency of his own convictions by casting his own vote against the bill, modernise his image all the same by allowing same-sex marriage to take effect on his watch, and perhaps throw a wedge into Labor for good measure, as it prepares to debate a binding vote on the matter at its national conference in July.