The move comes as the party's Trevor Lunn announced at an event in Belfast that he would now support gay marriage.
The Lagan Valley MLA told the BBC a number of things had changed his mind.
"The vote in the south which is certainly very impressive - a very conservative country," he said.
"I've been certainly heavily lobbied by the various groups and friends - I've lots of gay friends.
"The big rally in Belfast, which I thought was very impressive.
"Overall, I have decided it's time to get a bit progressive, perhaps, and fall into line with this measure."
Welcoming the development, gay and lesbian support group the Rainbow Project said: "Trevor has long been a supporter of the LGBT community in Northern Ireland and has been a strong voice for the rights of LGBT young people in education.
"We are enormously happy that Trevor has now come out in favour of equal marriage."
On Saturday, Alliance Leader David Ford said he was delighted to see party members come out in force to support this year's Belfast Pride.
Mr Ford was joined by a number of Alliance elected representatives during the parade.
The Alliance Party has said it will make a commitment of support for same sex marriage part of its manifesto for the next Assembly election.