What social policy of the democrats is deeply unpopular? Explain?
It isn't that one thing is especially unpopular. Marijuana legalization is popular at the national level at this point, but if you based the majority of your campaigns, energy, and effort towards pot issues you'd probably not do particularly well nationally.
The Democrats' raison d'etre is the promotion of sexual diversity, to the marginalization of issues that effect a wider swathe of the public. The Democrats should be winning every cycle by enormous margins given the fact that most people agree with them on important issues.
How many Democratic candidates made raising the minimum wage the lynchpin of their campaign? Half the country LIKES Obamacare and 25% of the people opposing it thought it didn't go far enough. How many Democratic candidates in swing states actually ran on it, or even suggested we use it as a starting point towards single payer? The great majority of the country wants immigration reform, but the Dems shrank away from it. If the Democrats cared about young voters (who didn't show up at all yesterday) or single voters they would seriously be proposing some kind of national student debt forgiveness. They play games with the numbers to make it seem like the economic equity and the structural foundations of the economy aren't in horrific shape in our country, and think that people will be satisfied with promises of free birth-control and abortions.
The funny thing is that the Democrats don't have to have nights like last night. They already have the voters out there, they just need to get them to the polls. If they actually spoke to issues people cared about and ran on the obvious solutions to those problems instead of half-assed cop-outs and excuses, they wouldn't have to worry about turnout either.