The AP article provides a good overall situation summary:
The Netherlands, Canada, Belgium and Spain have legalized same-sex marriage, while several other European countries have laws giving same-sex couples the right to form legally binding civil partnerships. In the U.S., only the state of Massachusetts allows gay marriage, while Vermont and Connecticut permit civil unions
They leave out New Jersey, although the law hasn't actually changed yet, so that's accurate enough.
In Massachusetts, law-makers successfully used parlimentary procedures to prevent a referendum on a constitutional amendment to ban new gay marriages. Interestingly, the amendment that would have voided marriages that have already taken place was defeated unanimously. I'm no expert on Massachusetts constitutional law, but I'm a bit dubious of this approach. It seems like there's an intentionally low threshold for constitutional referendums. All you need is 50 of some 400 legislators to support it. However, the constitutional convention can be recessed by a simple majority vote. So the majority can easily block the 50 legislators. This seems like a pretty dumb system because it encourages dodging rather than winning votes. By comparison, the filibuster in the US Senate at least makes you work rather hard to dodge a vote.
Procedural issues aside, I think they should have allowed the referendum and then poured resources into winning it. It would be an after the fact popular democratic validation of Gay marriage. Yes, there's risks it would fail. But popular validation would be a critical counterarguments to complaints about judicial fiat. Moreover, if a pro-Gay marriage referendum was blocked using this kind of tactic, I know I'd be pissed.