I should certainly watch/listen to what Sarah and Suzie have to say here first, but the very question is weird to put it nicely. Even with the very dramatic change in public opinion on gay marriage, abortion is still more popular (and depending on how the question is phrased, it is much more popular). On top of that, there are some pretty good reasons why opinion on abortion wouldn't change much over time: while there really isn't any coherent reason to be opposed to gay marriage, there is to abortion (I may disagree with it, but the reasoning is sound).
The better question is that why, with abortion being more popular with the public, are politicians so hesitant to get behind it? And why, in particular, do nominally pro-choice Democrats keep giving in to the Neanderthal position on the issue? (That is probably not fair, I would guess that Neanderthals would be far more sympathetic to women's authority over their own bodies than are most Republicans.)