I think it's interesting to compare this to approval of inter-racial marriage. A couple of years ago I compared the rate of approval of inter-racial marriage to that of approval of marriage between same sex couples, using data from different polls*.
I've revised the graph with the newest poll numbers and added some landmarks.
I want you to notice a couple of things, as I've noted before. First, at the time of Loving v. Virginia, in which the Supreme Court overturned anti-miscegenation laws and declared inter-racial marriage legal, only 20% of Americans approved of such marriages.
Compare this to the 27% percent who already approved of marriage equality when DOMA passed, which prevented the recognition of same sex marriages. 20% approved when we legalized inter-racial marriage, 27% approved when we essentially de-legalized same sex marriages.
At the time marriage equality for gay couples was approved in Massachusetts (2003) about 35% of Americans approved. It took 10 years after the Loving decision for the numbers to reach that point. That is, only 10 years AFTER inter-racial marriage was legal, did that many people agree with it.
We're now just below 50% on approval of same sex marriage (sometimes a few points up, sometimes a few points down). It took 25 years AFTER inter-racial marriage became legal, to reach that point. So at the same point of approval of inter-racial marriage that we're at now, it had already been legal for 25 years.
Why are we still fighting this battle?
*Method: For this analysis, I assigned a starting year to the absolute value "0" (1968 for inter-racial data). Each data point was then plotted based on how many years had elapsed since year "0". I then registered the same sex marriage data against the inter-racial marriage data based on the nearest value (that is, 1996 for same sex marriage was closest to 1972 for inter-racial marriage). The density of points for same sex marriage relative to the inter-racial marriage reflects the difference in frequency of polling the questions.
*Inter-racial marriage from two Gallup polls here and here
*Same sex marriage from two Pew polls here and here
More Pew data analysis here