In June 2015, the US Supreme Court ruled it is a
Now, the lead plaintiff in that case, Jim Obergefell, is worried President-elect Donald Trump's victory will mean there is a greater chance the ruling could be overturned.
"Is the possibility there? Absolutely," Mr Obergefell told SBS. "And it comes down to Trump being able to nominate justices to the Supreme Court - and he has said over and over that he would nominate judges that would be willing to overturn marriage equality."
In President-elect Trump's first full television interview since his shock win last week, the Republican said that same sex marriage was "settled" and that it was "fine" by him.
"It doesn't give me any comfort to hear him say it's settled law."
"I don't know what to actually believe, (what) Trump actually thinks or believes because he changes his story on a regular basis," Mr Obergefell said.
"He says one thing one day and the next day he says something else - and then says he never said it the first time.
"Look at Roe v Wade, the right to choose - that's been settled law for over 40 years and yet they're going after that," he said.
Mr Obergefell said the bigger issue was Trump's running mate, Vice President-elect Mike Pence, who Mr Obergefell branded "the most anti-LGBTI politician in the country".
As Governor of Indiana, Vice President-elect Pence passed laws legalising LGBT+ discrimination on religious grounds and has voiced support for 'gay conversation therapy'.
Pence will be backed by a Republican majority in the Senate and House of Representatives, and the party's platform remains opposed to marriage equality.
"Regardless of what Trump says, the people that can actually propose and vote on laws - they are against us," Mr Obergefell told SBS.
"It doesn't give me any comfort to hear him say it's settled law."
