Australians are pinching themselves, not least Abbott's sister Christine Forster.
"That will be for me one of the proudest moments of my life," she says.
Why? The mother-of-four is gay and lives with a long-time partner she met while they dropped their sons off at preschool. The wedding her big brother will make legally possible should be quite a sight, if only for the presence of the Prime Minister and drag queen bridesmaids.
Not that Abbott's position has wavered. "I do think that a marriage, by definition, is between a man and a woman," he has said in the past. It's a principle he continues to hold dear.
But Abbott - and the majority of Canberra's political class - have been overtaken by events.
Until this week they had been deaf to rapidly evolving community sentiments and widespread acceptance of marriage equality.
But after the Irish voted overwhelmingly in favour, momentum for change on the other side of the world is proving unstoppable.
Previously opposed politicians are coming out in favour. Right-wing shock jocks are changing their tune. Yesterday dozens of big companies took out a full-page advert in the Australian to pledge support.
"In New Zealand there was a turning point when conservative radio hosts came out in support of reform and that's what we're seeing happen here," NSW state MP and gay rights campaigner Alex Greenwich told the paper. "They are able to reflect the sentiment of areas like western Sydney and it encourages people to come on board with the reforms."
Abbott was stung into action by Labor opposition leader Bill Shorten and deputy Tanya Plibersek, who flagged the introduction of a private member's bill.
Their Marriage Amendment (Marriage Equality) Bill 2015 replaces "man and woman" and "husband and wife" with "two people", but doesn't legally require churches, ministers or celebrants to solemnise same-sex marriages.
Abbott accused Labor of opportunism.
"If our Parliament were to make a big decision on a matter such as this, I want it to be owned by the Parliament, and not any particular party," said the PM.
"There are very different views inside the Parliament, inside the parties, indeed inside families, as is well known in my own case."
Marriage equality was last debated and comprehensively defeated by federal parliamentarians in 2012.
Unlike then, when the leadership of the Liberal National Coalition ordered MPs to vote against, they are now likely to be allowed a conscience vote. Abbott has indicated he will support a free vote if that is the wish of the majority of his party.
Labor is committed to a conscience vote, although some in the party, like deputy leader Plibersek, support binding MPs to voting for reform.
Although Shorten and Plibersek intend to press ahead on Monday, their bill seems doomed to fail.
More likely is the introduction of a cross-party bill sponsored by backbenchers from both main parties, and possibly the Greens. It would most likely be debated and voted on after Parliament resumes from the winter recess in August.
Although support is said to be evenly poised in both the House of Representatives and Upper House Senate, Abbott's shift in rhetoric has warmed progressive hearts.
What was once an inner-city preoccupation and illegitimate distraction from serious issues is now an "important issue".
That's music to the ears of his sister, who says Abbott - despite his beliefs - appreciates the desire for change.
"At the end of the day I'm sure he will understand that this is good and fair for everybody and it will be great for Australia," she told the ABC.
Asked if she would marry partner Virginia Edwards, Forster replied: "I haven't checked in with her lately but last time we discussed it she was still game on."
