We (culturally) have thousands of years of social and legal experience with heterosexual marriage. It's a very complex institution. Making a radical change to it may have unintended consequences.
My view is that allowing gay marriage is itself a consequence (probably unintended but still welcome) of other radical changes to the institution of marriage in Western society in the last century or two. Such changes include the view of a marriage as a personal unit rather than an economic one united by love rather than mutual economic and social benefit, and the general increase of women's rights and convergence of gender roles (for example, women can own property, and are not treated as being property themselves). The whole "soulmate" thing is a relatively recent idea, but is the current ideal for marriage.
We have thousand of years of experience with a variety of institutions that share the name "marriage" but aren't very similar to each other. The current institution that bears that name has arguably only been around for about forty years, most recently being re-written by the sexual revolution and the increasing social acceptance of divorcing and re-marrying. Considering how flexible we as a society have become with respect to gender roles, this change is hardly "radical" by comparison.
No, actually we don't. Marriage throughout history had very different social and legal rules. Inheritance rules, power, authority, and care-taking of children are elements that have changed radically over the years.
It would be accurate to say we've had experience with many different kinds of heterosexual marriage, but once you're lumping in, say, 16th century English marriage laws and customs with 21st century Californian ones, gender starts to look like a minor detail.
We talk about "marriage" as if it's a simple, single idea, but historically, it's been applied to a wide range activities.
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.
