An interesting blog post from the economics editor at theAtlantic -

From the Lifestyle editor of GOOD:
The institute calls this a "decoupling of attitudes." Support for same-sex marriage and abortion rights have traditionally gone hand-in-hand, and that's changing. Though young people today are "more educated, more liberal, and more likely to be religiously unaffiliated" than their parents--all factors traditionally correlated with support of abortion rights--they are not actually more likely to support abortion.

Here's one explanation for the decoupling: Youth support of same-sex marriage does not reflect an embrace of progressive values, but rather an expansion of conservative ones. Over the past several decades, the mainstream gay rights movement has aligned its priorities with fundamentally conservative institutions: Gays and lesbians want the right to get married, adopt children, and serve in the military. These family-friendly, all-American demands appeal to the conservative base, and work in direct contrast to the lingering stereotyping of gays as promiscuous Communists. Today, support for gay marriage is nearing 50 percent among even the most conservative of American youth, likeRepublicans and white evangelical Christians.

Reframing abortion rights as a family value is a trickier sell. Though about one-third of women will abort a pregnancy in their lifetimes--a figure that spans age, ideology, and religion--many who have undergone the procedure remain in the closet. And the increasing acceptance of gay marriage among conservatives may not help the cause. After all, the anti-abortion movement has now found a key ally in gay Republicans.

From Megan McArdle - who thinks there's a simple explanation - and it comes down to the idea of "harm":

And yet the article doesn't come off like she's trying to hide the football. She's really trying to explain why those Cro Magnons in the millennial generation aren't increasing their support for abortion. It apparently doesn't even occur to her to think that while the harm from gay marriage is pretty nebulous, the harm from abortion is pretty obvious to everyone. (Whether or not you think it's important enough to ban is of course an entirely separate question . . . but I think it's fair to say that everyone recognizes that something unpleasant happens to some sort of organism that absent the abortion, will almost certainly turn into a human being.)

Instead, she apparently thinks that opposition to abortion/abortion rights comes from conservative allegiance to an ill-defined concept of "family values", support for which gets turned on or off like a lightbulb. If this were true, it would of course make the decoupling of support for gay marriage with support for abortion really surprising. But given fairly lopsided views on the matter until recently, its probably more accurate to say that support for gay marriage used to come from allegiance to a somewhat nebulous concept of "sexual freedom" or "progressivism" and that as it's gone mainstream, it's simply gaining support from people who still don't support those totems any more than they ever did.

The absence of that allegiance to those value constellations does not imply support for their inverse--thinking that it does is a classic fallacy called egotistical bias. I doubt that most people in America think about the issue in terms of political allegiance. I'm guessing most of the people who changed their minds did so because gays seemed to want it, and they couldn't really see the harm.