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 ECJ's advocate general, Melchior Wathelet, said in an official legal opinion there had been "evolution" in the societies of EU countries, and the idea that "the term marriage means a union between two persons of the opposite sex can no longer be followed".
Romania does not have marriage equality - but it must now recognize the rights of same-sex spouses.
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.
"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".
So while the 28 EU countries "are free to provide or not for marriage for persons of the same sex", they must not limit their application of spousal rights in a way that infringes "on the right of citizens of the Union and their family members to move and reside freely within the territory of the Member States", Wathelet wrote.
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"Freedom of movement is a right of all European Union citizens, it can not be restricted because of whom they love".
The Alliance of Romanian Families and the Coalition for Family, two conservative Christian groups which oppose same-sex marriage didn't immediately respond to requests asking for comment on the development.
Wathelet issued the opinion in regards to a 2010 case involving a Romanian citizen named Adrian Coman who married his American husband, Claibourn Robert Hamilton, in Brussels in 2010. The men appealed to Romania's constitutional court, which referred it to Luxembourg.
But the Romanian government has so far refused to grant Hamilton a residency permit. He explained that European Union law was neutral on the gender of a spouse. Like Romania, Poland, Slovakia, Bulgaria, Lithuania and Latvia do not give same-sex couples any legal rights or responsibilities.
The EU's freedom of movement principle grants rights to an EU citizen's spouse, without specifying how spouse is defined. "Statistical investigations confirm it; the authorization of marriage between persons of the same sex in a referendum in Ireland also serves as an illustration".
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.