I think there were two elements. One was that gay marriage suddenly started happening in many places around the world and instead of dire warnings coming true, all the public heard were media reports of some happy couples getting married. The other was generational shift: a generation who had grown up when homosexual relations were unthinkable where even many left-leaning people felt the natural order was being disturbed and Something Must Be Done was supplanted in numbers and political influence by a younger generation where even most conservatives didn't exactly see the gay people they knew as a threat. (And in some parts of the world, the Right even flipped the whole thing on its head and used "their homophobia makes them not like us" as an argument in their campaigns against immigrants or religious minorities). In the middle, lots of people's views on marriage turned out to be a lot less strong than the conservative culture warriors had hoped...

I think it goes to show that conservatives can generally rely on younger generations becoming more receptive to their arguments about tax and security and Chesterton's Fence (and less interested in provisions for people on low incomes) as they age to compensate for older conservatives dying off, the switching sides in culture wars tends to trend in the opposite direction.