Here's part of the story and recent chart from Nate Silver at the New York Times:
The availability of same-sex marriage is increasing almost as rapidly on a global scale. It was legalized in Brazil and France earlier this year and will become legal in Uruguay and New Zealand by August.
A decision last month by a Brazilian judicial panel that is generally seen as legalizing same-sex marriage, but could be subject to appeal, is especially important to this math. Brazil has a population of about 194 million — more than the combined 169 million in the nine countries in Europe where same-sex marriage is now legal. The most recent decision followed court rulings that had authorized same-sex marriage in more than a dozen Brazilian states.
Earlier this year, France, with a population of about 64 million, became the largest European country to legalize same-sex marriage, and the largest in the world to do so by legislative action.
By August, there will be about 585 million people living in countries or jurisdictions where same-sex marriage is legal.