Here's a contrast: At the same time the Supreme Court held oral arguments on a case that could legalize same-sex marriage, North Dakota lawmakers passed one of the most restrictive abortion bans in the nation. It's a sign, argues Sarah Kliff in The Washington Post, that the two have decoupled as issues of controversy, "Younger Americans have become increasingly supportive of gay marriage in a way that hasn’t necessarily happened for abortion rights."
On the whole, "Millennials" are just as ambivalent on abortion rights as their older counterparts. To wit, only 50 percent of Americans under the age of thirty believe abortion should be legal in all or some cases, compared to 54 percent of Americans in their 30s and 40s, and 55 percent of Americans in their 50s and 60s.
But is this evidence of a "decoupling" of the two issues? In terms of public opinion, the information is clear—it is. But polls aren't the same as political coalitions, and it's harder to say the parties have changed their relationship to the two issues. The strictest abortion regulations, for example, are in Republican states—like Oklahoma, Kentucky, and Kansas—that have also banned same-sex marriage. This map, from Talking Points Memo, is useful:
With few exceptions, the red states are also states with strict abortion regulations, and the blue states are also ones with the most access to abortion and other reproductive health services. Likewise, politicians from the red states tend to be anti-abortion and opposed to marriage equality, while politicians from the blue ones tend to support abortion rights and same-sex marriage.
Writ large, neither party has moved from its usual position on both issues. The Democratic Party remains committed to the status quo established by Roe v. Wade and Casey v. Planned Parenthood, while the GOP seeks to overturn the former and pass laws—on the state and federal level—that criminalize abortion. And while the Democratic Party has taken a rapid turn towards support for marriage equality, most Republican politicians—a substantial portion of Republican voters—still oppose same-sex marriage, even if they aren't vocal in their opposition (to say nothing, of course, of GOP activists, especially in the evangelical community).
As far as I can tell, neither party has changed as far as their positions on either issue. At most, Democratic supporters of same-sex marriage have become far more vocal on the issue, while Republican opponents of abortion have done the same. Whether this stays true, of course, is an open question.