A final decision will be delivered in the coming months.
The case involved Romanian Adrian Coman and American Clai Hamilton, who married in Brussels in 2010.
A senior adviser to the European Union's top court has backed a Romanian gay man's right to have his U.S. husband live with him in Romania.
The role of the 11 advocates general is to propose independent solutions for cases the Court of Justice is deliberating on.
Wathelet added that "the concept of "spouse" within the meaning of the directive also includes spouses of the same sex". 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.
"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.
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If Coman's bid is successful, the ruling would be controversial in Romania, where USA evangelicals have pushed a law to ban same-sex marriage.
Giving his opinion, Wathelet said governments may not impede the freedom of residence of an European Union citizen by refusing to grant his or her spouse, who is a national of a non-EU country, a right of permanent residence in their territory.
Despite their legal marriage in a fellow European Union state, Romania is resisting the activist's attempts to gain residency rights for his husband, as it does not allow same-sex marriage.
Basically, Wathelet's opinion lays the groundwork for a landmark LGBTQ rights victory that would affect Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden and the United Kingdom.
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. He explained that European Union law was neutral on the gender of a spouse. It's believed that improving the rights of same sex couples in law will bring a wider improvement of their rights across the EU. It will also ensure the term "spouse" is always seen as gender neutral by law.
It was his opinion that "in view of the general evolution of the societies of the member states of the EU in the last decade in the area of authorisation of same-sex marriage" it was no longer appropriate to follow the case law definition of marriage as "a union between two persons of the opposite sex".