Not to mention the population of the US is almost larger than every other country on that list combined.

It's an astounding feat to have accomplished such a vast cultural change in such a short amount of time. I've read recently that upwards of 25% of the entire US population has shifted its view on gay marriage in just ten years.

GP did specify "the developed world". It is a very fuzzy criteria, but however you slice it it's a far cry from 195 countries. The IMF identifies 37 advanced economies and the World Bank lists 31 "high-income OECD members" and 28 Development Assistance Committee members.