New York currently does not have a referendum/initiative provision in its law - but a bill is moving through the legislature.
When it is passed, as is expected, only 5% of the electorate needs to sign an anti-gay marriage petitionin order to get it on the ballot.
Then, I think it only needs to be passed by a simple majority in order to repeal the law ...
I would think that this would require an amendment of the state Constitution.