The 60-year-old Victorian Labor MP announced his new position on Monday - his first day back at work after suffering a heart attack in April - acknowledging that he had previously voted in favour of “the traditional definition of marriage”.
He is the latest politician to change their stance on marriage equality, with a number of prominent politicians such as Chris Bowen and Ed Husic now in favour. Wayne Swan has also changed his position to support same-sex marriage, along with Tony Burke.
Citing the recent Ireland referendum to legalise same-sex marriage, Mr Thomson said it was time for his position to reflect the public’s changing attitude.
Recommended Reading
Interactive: Where does your local politician stand on same-sex marriage?
“Since then it has become apparent to me that there has been steadily increasing community support for same sex marriage,” he said.
“This is evident in the conversations I have with constituents and the representations I receive from them. It is evident from a number of my Labor colleagues - Wayne Swan, Ed Husic, Chris Bowen, Tony Burke, Julie Owens, Bernie Ripoll and Joel Fitzgibbon - who like me voted against amending the Marriage Act in the last Parliament, now indicating that they support same sex marriage.”
Mr Thomson also called for a conscience vote on the issue within the Liberal Party, which has previously been denied by Coalition leadership.
His comments came on his first day back in parliament after a heart attack almost two months ago, though he hinted at his position in a Facebook post last month.
The private member's bill to legalise same-sex marriage introduced by Opposition Leader Bill Shorten and Deputy Tanya Plibersek will need a total of 76 votes to pass.
Advocacy organisation Australian Marriage Equality believes the bill has a near majority in the lower house, with at least 72 votes, while SBS has independently confirmed 60 votes. An additional two believed to be in favour.
These have been calculated on the basis that the Coalition will be granted a free vote, otherwise the bill is doomed to fail under its 89 vote majority.
Some politicians contacted by SBS have been unwilling to declare their position publicly.
