Mr Coman, a Romanian national, and Mr Hamilton, a USA national, cohabited for four years in the United States before marrying in Brussels in 2010.
"Although member states are free to authorize marriage between persons of the same sex or not, they may not impede the freedom of residence of an European Union citizen by refusing to grant his or her spouse of the same sex, a national of a non-E.U. country, a right of permanent residence in their territory", said Melchior Wathelet, advocate general of the ECJ.
"Romania's Constitutional Court recently approved and confirmed more than three million citizen signatures (in a country of less than 20 million people) calling for the national referendum".
"Although member states are free to authorise marriage between persons of the same sex or not, they may not impede the freedom of residence of an European Union citizen by refusing to grant his or her spouse of the same sex, a national of a non-EU country, a right of permanent residence in their territory", he said.
The couple's case is giving the European Court of Justice its first opportunity to consider if an EU directive on the rights of citizens and their family members to "move and reside freely" within the bloc applies when married spouses are two men, according to Wathelet. The couple met in NY in 2002 and lived together there for four years, but Hamilton remained in NY initially when Coman moved to Belgium for work.
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But because Romania does not recognise same-sex marriage, Hamilton's bid for a residence permit to live there was refused.
One of the EU's most senior legal advisers has said that same-sex partners of European Union citizens enjoy the same free movement rights as partners of a different gender.
The ECJ has had a relatively benign history with the United Kingdom - unlike the more contentious European court of human rights in Strasbourg - but nonetheless became a symbol of compromised sovereignty during the Brexit referendum. The couple challenged the decision in Romanian court, claiming it was discrimination based on sexual orientation. In addition to Romania, other E.U. countries that have no legislation recognizing same-sex unions or marriages include Poland, Lithuania, Latvia, Slovakia, and Bulgaria. In that regard, the Advocate General points out that the term "spouse" within the meaning of the directive refers to a relationship based on marriage while nevertheless being neutral as to the sex of the persons concerned and indifferent as to the place where that marriage was contracted.
Wathelet maintains that European Union law defines "spouse" includes legal married partners of the same sex and makes no reference to member nations' definition of the word.
Opinions given by ECJ advocate generals are non-binding on the court's judges but are normally followed by the full court.