When they returned to live in Romania, the Romanian government replied that it doesn't legally recognize same-sex marriages.
This week, ECJ advocate general Melchior Wathelet said that should include same-sex spouses who Wednesday in a country that has equal marriage on the books.
"The term "spouse" includes, in the light of the freedom of residence of citizens of the European Union and their family members, spouses of the same sex", Wathelet said.
But because Romania does not recognise same-sex marriage, Hamilton's bid for a residence permit to live there was refused.
The case involves a Romanian male national who had married a man from the United States in Brussels in 2010. Accordingly, such a person may also reside on a permanent basis in the territory of the member state in which his or her spouse is established as an European Union citizen after exercising his or her freedom of movement.
Wathelet wrote that the definition of marriage being between two persons of the opposite sex was becoming outdated, referring to "the general evolution of the societies of the member states of the European Union in the last decade in the area of authorisation of same-sex marriage".
"That conclusion also applies in respect of that citizen's country of origin, when he returns there after residing on a permanent basis in another Member State in which he has developed or consolidated a family life, as Mr Coman has done with Mr Hamilton in the present case". The couple challenged the decision in Romanian court, claiming it was discrimination based on sexual orientation.
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Advocates general's opinions are not binding, but the court often rules the same way as the advocate general.
Sophie in 't Veld, the Vice President of the European Parliament's LGBTI Intergroup, told PinkNews: "This is fantastic news and a landmark opinion for rainbow families!" The EU has insisted that any future agreements on citizens' rights and access to the single market must continue to be policed by the ECJ. The country does not recognize any form of legal union between those of the same gender.
"Granting the spouse of a union citizen a right of residence constitutes recognition and the minimum guarantee that can be given them", the opinion concludes.
Romania decriminalized homosexuality in 2001 but prohibits marriage between people of the same sex, and does not recognize same-sex marriages carried out overseas.
In a message to supporters, the group bragged about its interventions in the eastern European country in a bid to secure a ban on same-sex marriage.
Coman added: "Romanian citizens can't be divided into good and gay". Romania's constitutional court then referred the case to the ECJ.