The significance of this Washington Post/ABC poll showing a majority of Americans in support of marriage equality is really that it's only the latest to do so. Several other polls have shown similar results, which suggests a very real shift in public perception toward acceptance.
There are enough people opposed, and enough people who are motivated solely by issues like this one to give anti-gay-rights folks a few more years of successes at the ballot box, whether we're talking politicians, judges, or referendums. But at this point, they have little hope of reversing the direction of public opinion. They are trying to sandbag a tidal wave.