Easy. Atheism.
I read an analysis once that argued all the foofraw over abortion was due to legalization via decree, rather than legalization via legislation. The gist was that Roe V. Wade never gave society a chance to come to grips with abortion, thereby setting the stage for skirmish after skirmish over the ensuing 40 years. From that standpoint, the prevalent, recurrent racism throughout the United States in the 150 years since the Civil War are an outcome of slavery being passed as a gambit to ruin the south as opposed to a firmly-held social belief.
On the one hand, I see the point. There hasn't been any gay marriage court ruling and over the past 10 years it's become more legal than not. On the other hand, I suspect that slavery would still be de-facto legal if the Civil War hadn't been fought. After all, citizenship in Japan isn't that far off from Dred Scott. The US is racist but it's nothing next to the rest of the world.