I am not one to advocate that activism in the arena of procuring equal rights for non-heterosexual individuals should be entirely negated. It very well could be that the reason for the obvious trend in this graph is that activism and outreach programs are having a positive impact on the public perception of marriage and love between humans. It could also be the case that the waning impact of organized religion in younger generations has a compounding effect on these perceptions. I have yet to encounter a rational argument against homosexual marriage rights and on top of that I have yet to encounter any argument against it at all that does not have a foundation in theistic ideals and assumptions (take a look at Utah on this graph). All of this, however, is a bit tangential to the real crux of the graph here. What we are seeing is a distinct trend in America where the younger generations are not as subject to bigotry and discrimination.
I'm pretty stoked for the future.