Coming from a European, I think that's a bit of unnecessary quibbling intended to justify condescending to the Americans. The United States started out as something like the European Union: a partial union of sovereign countries. As such, the appropriate comparison would be to ask: where's the ruling by the European Court of Human Rights declaring that all EU members must implement gay marriage, or else? Get with the times, you backwards-looking conservatives on the Old Continent!
And, to this day, the separate regions, which are admittedly larger than states, actually do behave somewhat like different countries. The differences become especially visible when you look at public policies and development indices: different states in the USA have various levels of honesty in government, civil rights, economic development, average education, etc. In fact, on a state-by-state basis, some US states are more highly developed than many/most European countries.
So: Netherlands, Belgium, Massachusetts.