"Married registered voters prefer Republican challenger Mitt Romney over Democratic President Barack Obama by 54% to 39%..."

Says Gallup (looking at June-August data):

On the other hand, nonmarried voters break strongly for the president over Romney, 56% to 35%. Marriage is a significant predictor of presidential vote choice even after income, age, race, gender, education, religiosity, region, and having minor children are statistically controlled for.