(Since today's pride day in Seattle I figured this was an appropriate topic. I've been reflecting on this for a week or two so it's not exactly a well-formed thesis, but I'm curious how people think.)
I really feel like "change" is not the right word for social progress. Sure, it's technically accurate—it was a change in the Revised Code of Washington in the past couple years that allowed two people of the same sex to get married here.* But it implies, to me at least, that something's different: that the terms of morality have changed, or that it's some radical new idea, or that it's just another option and we decided to pick a different one. And I don't feel like it's any of those things. Maybe I'm reading too much into the word.
Treating women as second-class citizens and not allowing them to vote was never okay. To call it a "change" when women were given those rights feels like whitewashing over a grievous injustice. Similarly (and perhaps with a little hyperbole), slavery was never okay, arresting and killing gay people was never okay, and rounding up Jews into ghettos and concentration camps was never okay. It just became increasingly difficult for people to justify those things to their neighbors. A person in 1714 who owns ten slaves not a better person than someone in 2014 who owns ten slaves; he just had a lot easier time justifying his atrocities. We're rapidly progressing to a point where more and more people agree that it's not acceptable to deny to gay people the rights that straight people have, and where more and more people will look back on the humans of 2014 and wonder why we were all so horrible.
Hopefully we'll all feel that way someday about societies that don't provide healthcare for the sick, and food for the hungry. If the people of 2114 look back on the people of 2014 and are as ashamed of us for giving medicine to only those with money as we are of the slave owners in 1850, that's a good thing—then future generations have progressed to a point where the awful things we do (or don't do) today are no longer socially acceptable.
(Really, this largely comes down to morality versus ethics, a philosophical topic that bores me about as much as every other philosophical topic, which is perhaps ironic given that I just wrote this post. I'm a firm believer in morality and right and wrong. Our society's ethics allow behaviors that, to me, are quite immoral, but over time we seem to be slowly closing that gap.)
*And as a side note, I'm similarly annoyed by the phrase "gay marriage," where I'd strongly prefer "marriage equality." I don't like the implication that it's this new, separate thing; it's fixing an old thing that was broken, and making it fair.