The photo caption listed Melania and eight other first ladies in attendance but failed to name Gauthier Destenay, the husband of Luxembourg's gay Prime Minister, Xavier Bettel.
Destenay married Luxembourg Prime Minister Bettel in 2015.
After drawing condemnation across social media for the "embarrassing" and "disrespectful" omission, the caption was updated to include Destenay, 10 hours after it was originally posted, the ABC reported.
When the photograph, taken by Andrea Hanks, was initially posted on the White House Facebook page Destenay's name was missing from the caption.
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While some suggested the omission was innocent, many slammed it on social media as "embarrassing" and "disrespectful", the report said.
The photo was taken at the last week's summit in Brussels and was posted on Facebook in a collection of images from President Donald Trump's nine-day international tour.
There was no indication from the White House why Destenay's name had initially been left out of the caption.
She married writer Jonina Leosdottir in 2010.
Trump had previously stated he is "fine" with the legislation passed during the Obama administration which allowed couples of the same sex to marry, however Vice- President Mike Pence is known to have strong views in opposition to same-sex marriage.
In a 2006 speech as head of the Republican Study Committee, Pence drew links between marriage law reform and "societal collapse" and claimed refusing to allow gays to marry was not discrimination but an upholding of an idea "ordained by God".