As President Trump met with world leaders at the NATO summit in Brussels on Thursday, first lady Melania Trump gathered with nine other political spouses for a "family photo" near the steps of the Royal Castle.

In the front row, the first lady stands shoulder-to-shoulder with Brigitte Macron, wife of the newly elected French President Emmanuel Macron; Emine Gulbaran Erdoğan, wife of Turkey's President Recep Tayyip Erdoğan; Queen Mathilde of Belgium; Ingrid Schulerud, the wife of NATO's Secretary General Jens Stoltenberg; Desislava Radeva, the partner of Bulgaria's President Rumen Radev; and Amelie Derbaudrenghien, partner of Belgian Prime Minister Charles Michel.

(Front row, L-R): Brigitte Macron of France; Emine Gulbaran Erdogan of Turkey; first lady of the U.S. Melania Trump; Queen Mathilde of Belgium; Ingrid Schulerud, wife of NATO's secretary-general; Desislava Radeva of Bulgaria; Amelie Derbaudrenghien of Belgium; (Back row, L-R): Gauthier Destenay of Luxembourg; Mojca Stropnik of Slovenia; and Thora Margret Baldvinsdottir of Iceland pose for a family photo on May 25, 2017, in Brussels. Yorick Jansens/AFP/Getty Images

The internet, however, isn't talking about the front row.

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Rather, the gaze of social media users across the globe went straight to the man in the photo's upper left corner. His name is Gauthier Destenay, he's an architect, and he's the First Gentleman of Luxembourg. He is also the husband of the world's only openly gay prime minister, Xavier Bettel.

(Front row, L-R) First Lady of Turkey Emine Gulbaran Erdogan, First Lady of the US Melania Trump, Queen Mathilde of Belgium. (Back row, L-R) First Gentleman of Luxembourg Gauthier Destenay, partner of Slovenia's Prime Minister Mojca Stropnik and First Lady of Iceland Thora Margret Baldvinsdottir on May 25, 2017, in Brussels. Yorick Jansens/AFP/Getty Images

When Bettel was elected Prime Minister of Luxembourg in 2013, he and Destenay were not yet wed. They were, however, openly in a civil partnership.

The two married in 2015, a year after their small, Roman Catholic country legalized same-sex marriage. In doing so, Bettel became the first European Union government leader to enter into a same-sex marriage.

Luxembourg Prime Minister Xavier Bettel (R) waves next to his husband Belgian architect Gauthier Destenay during their wedding in Luxembourg on May 15, 2015. That day, Bettel became the first European Union leader to enter into a same-sex marriage, a symbol of growing social change across the continent. John Thys/AFP/Getty Images

Helping to pave the way, in 2010, Iceland's prime minister, Jóhanna Sigurðardóttir, married her partner, novelist Jónína Leósdóttir. Belgium's former Prime Minister Elio Di Rupo was also open about his homosexuality, coming out in 1996 when a reporter asked him if he was gay and he responded, "Yes. So what?"

Gauthier Destenay is also not the inaugural First Gentleman to appear in a photo of NATO spouses. Joachim Sauer, husband of German Chancellor Angela Merkel, has appeared in similar photographs before. He simply was not present for this one.

So Destenay stands proudly as the lone male spouse in the photo -- a fact, which understandably, has the internet abuzz.