Christina Wolbrecht examines various possibilities:
[T]he gender gap is a persistent feature of American presidential elections, and has favored the Democrats since 1964. Why? Repeated research has failed to uncover evidence that women’s issues, including abortion, cause the gender gap. Long story short, there are few consistent gender differences in attitudes on such issues, and limited evidence that women prioritize women’s issues such as abortion in their voting calculus to a greater extent than men do. If and when such issues do influence vote choice, we have reason to believe they may work in either a liberal or conservative direction. Finally, as I show in my book, the parties only diverged on women’s issues in the late 1970s, and as we’ve seen, the pro-Democratic gender gap emerged earlier than that.
She instead attributes the gender gap to men moving away from the Democratic party. The reasons are complex but "a big part of the answer appears to be divergence in preferences for social welfare policies, such as aid to the poor and elderly, health care, and so on."
The Dish 
