Why everyone is talking about this photo from the NATO summit.

At the NATO summit currently being staged in Brussels, the spouses of some of the world’s most powerful leaders posed for a photograph.

Among them was FLOTUS Melania Trump, Queen Mathilde of Belgium and France’s new First Lady Brigitte Macron.

But everyone is talking about Luxembourg’s First Gentleman Gauthier Destenay.

He's not the only male NATO spouse, of course - German Chancellor Angela Merkel's famously publicity-shy husband, Joachim Sauer, was absent this year.

But the Belgian architect is one-half of the world's only openly gay first couple.

Gestenay has been married to Luxembourg's Prime Minister Xavier Bettel since 2015, the year same-sex marriage was legalised in the tiny European nation.

They've been together since 2010, and were in a civil partnership when the Democrat was elected to high office in December 2013.

“I have just one life, and I don’t want to hide my life,” Bettel previously told Los Angeles Times. “But I was not the ‘gay candidate.’... People didn’t vote for me because I’m gay or I’m straight.”

Bettel is the third openly gay world leader in modern history, alongside former Prime Minister of Iceland Jóhanna Sigurðardóttir and Belgium's Elio Di Rupo.