It clearly can be debated what is and isn't radical change.
However, "Allowing a tiny sliver of the population to enjoy the same privileges and responsibilities as the rest of us?" can be a radical change. For example, it's exactly what happened when we abolished slavery.
I'm not trying to equalize slavery with gay marriage, just to point out that something that fits your criteria can be considered a radical change.
As to gay marriage rights, given that a majority of US population is against it (as was proved in California, one of the most progressive states, with Prop 8), the fact that courts suppressed Prop 8 is pretty radical. It pushes the equality goalpost a little bit further.
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.
Let's be clear here: compulsory gay marriage would be radical. Allowing gay people to marry is not.