The LA Times has a nice interactive map of gay marriage rights by state that shows the many changes that have taken place over the past decade or so. Here's how things stood in 2000:
And here's where things stand today:
The scale represents the range of rights denied or afforded to gay couples, from "constitutional amendment ban[ning] gay marriage and other legal rights for gay couples" (dark red) to "domestic partnership legal" (pale green) to "gay marriage legal" (dark green). You can mouse ove states for details. They also have a timeline depicting the various votes, judicial rulings, and other events that have produced the civil rights hodgepodge depicted on this map.
On the face of it, it looks like there's been a lot of movement both for and against legal recognition for gay marriage; while some states have extended full marriage rights, others have retrenched with constitutional amendments banning the same. But I think it makes sense to see these as two sorts of steps that are actually fundamentally moving in the same direction. After all, prior to 2000 or so, marriage equality wasn't really seen as conceivable - not in the foreseeable future, at any rate. But then - possibly because of a genuine sense of a shifting ground in public sentiment - a bunch of states amended their constitutions. This was a defensive move which anticipated the possibility that gay people might be allowed to marry which, like most changes in social institutions, tended to freak out people with more traditionalist orientations. But even though the effect was that a bunch of anti-equality measures got written into law, this was a symptom of a general progressive movement on the issue of gay marriage.
In retrospect, I think you might even say that the big movement earlier this decade to amend state constitutions to explicitly ban gay marriage actually spurred further progressive movement. Those votes - many of which were orchestrated in an effort to get more people to come out and vote Republican in 2004 and 2006 - raised the profile of the issue, certainly, and perhaps placed a veneer of plausibility on the concept of gay marriage. After all, if it was a concept that had to be fought at the ballot box, it must have been a concept that needed to be taken seriously, right? And with several states approving gay marriage this year, following the high-profile loss of gay marriage rights in California last November, I wonder if all the efforts at thwarting gay marriage have just had the net effect of telescoping the timeline of the spread of marriage equality.
6/3 UPDATE: And New Hampshire officially becomes the 6th state to sanction gay marriage, and the third to do so legislatively, after Vermont and Maine. Rhode Island is the only New England state without marriage equality.