The Equality Network said 69 MSPs have signed its pledge saying they would back a change in the law.
They include the leaders of all the opposition parties, the majority of SNP and Labour MSPs, and all Liberal Democrat and Green MSPs.
Key Scottish Government cabinet ministers have also indicated they will be voting for the measure, including Alex Salmond, Nicola Sturgeon, John Swinney and Kenny MacAskill.
If the numbers translate to a vote in the Scottish Parliament, it would mean equal marriage legislation would be passed....