A few seconds after midnight on April 1 2001, the first gay weddings on earth took place in Amsterdam town hall. Four couples were simultaneously married by the city's mayor, Job Cohen. I grew up in the Netherlands, and re-watching the scenes, I found them that typically Dutch thing: sexual revolution, bourgeois style. The newly-weds exchanged the traditional kisses while ageing fathers in suits and ties beamed from the town hall's benches. When the couples signed the register, they looked just like the stolid burghers of 17th-century Dutch paintings, except gay.

Nobody suspected then that by 2013, half the western world would be following them. Already one in five Americans lives in a state with gay marriage, and now Barack Obama's government is asking the US Supreme Court to overturn a federal law banning the practice. The British and French parliaments voted for gay marriage in February. Yet in these countries, gay marriage is often debated as a leap into the unknown. It isn't. As with so many social issues, the Netherlands is a laboratory. Twelve years on from Amsterdam town hall, we have a pretty good idea of how gay marriage changes a society.

The first thing to note: it doesn't change much. Almost as soon as gay marriage was introduced, it faded from Dutch political debate. The unprecedentedly angry political arguments in the Netherlands since September 11 2001 have been about Muslims, Brussels and social class. Even when the Christian Democrats re-entered government in 2002, they never tried to ban gay marriage. Only one in nine Dutch people now opposes the institution, says the state's Social and Cultural Planning Bureau.