Today's xkcd tracks popular approval of interracial marriage and same-sex marriage and concludes:

People often say that same-sex marriage now is like interracial marriage in the 60s. But in terms of public opinion, same-sex marriage now is like interracial marriage in the 90s, when it had already been legal nationwide for 30 years.

The thing is, the polls aren't asking parallel questions. For interracial marriage the question tends to be "do you approve" while for same-sex marriage it's "should it be legal". For example, gallup asks:

Do you approve or disaprove of marriage between blacks and whites?

which crossed over in 1991 and:

Do you think marriages between same-sex couples should or should not be recognized by the law as valid, with the same rights as traditional marriages?

which crossed over in 2011.

I'm trying to find polls asking about legality of interracial marriage, approval of same-sex marriage, but haven't succeeded. [1] There are lots of same-sex marriage poll questions people ask, but they're pretty much all about legality:

• "Do you think it should be legal or not legal for same-sex couples to marry?"
• "Do you strongly favor, favor, oppose, or strongly oppose allowing gays and lesbians to marry legally?"
• "Overall, do you support or oppose allowing gays and lesbians to marry legally?"
• "Do you support or oppose allowing same-sex couples to get married?"
• "Do you favor or oppose allowing gay and lesbian couples to enter into same-sex marriages?"
• "Do you think it should be legal or not legal for same-sex couples to marry?"

One poll did ask "In general, do you support or oppose same-sex marriage" but I think most people probably interpreted this as still asking about legality. It would be really helpful if some polling firm would ask half their sample "do you approve or disaprove of marriage between same-sex couples" and the other half "do you support or oppose allowing same-sex couples to get married?" so we could see the split.

[1] Well, there's one 2012 poll (pdf) of "likely Republican" voters in Mississippi and Alabama that found 67% thought interracial marriage should be legal (21% against, 12% unsure). But overall this doesn't seem to be a question people ask.