The decision will likely be as significant as the 1967 US Supreme Court decision striking down laws banning interracial marriage. Back then, most people felt that it shouldn't be allowed, but today nobody gives it a second look. The same will happen over time.
(Incidentally, the ban on interracial marriage was overturned two years before I was born-- good thing too, otherwise my own parents' marriage would have been considered illegal).
The best part is going to be watching all the ultraconservative pundits go into a foaming frenzy over the decision. If Falwell and Thurmond hadn't already croaked, they probably both would have today. Yup, just would keeled over. I'm sure that the religious firebrands will make all kinds of pronouncements that this is a sign of The Apocalypse and that the end is nigh, and that Divine Retribution is at hand. Will be lots of fun to watch.
What happens in California happens to propagate to the rest of the country after a few years-- we'll see what happens.