Well either you are comparing the abolition of slavery to gay marriage or you aren't. The abolition of slavery was a radical change by any measure. The adoption of gay marriage has had no discernible impact in any country or jurisdiction where it has been tried. Except that a tiny vulnerable minority of the population is a whole lot happier and more secure. The rest of us haven't noticed a thing. It's just not that big a deal, except for the people who need and deserve it.
Repliers: don't bother citing the contradictory NOM study. If you believe anything they have to say, there is no room for conversation between us. The study hinges on an argument from Tony Perkins' Family Research Council, which is about as biased as any organization can be.