The finding involves the case of Romanian national Adrian Coman and his American partner, Clai Hamilton, who tied the knot in Brussels in 2010 but have since been denied the right to live in Romania together. A few years later, Romania denied Hamilton spousal residency rights, however, arguing that he could not be considered the spouse of an European Union citizen given that Romania does not recognize same-sex marriage.
Quoting the European Convention on Human Rights and the EU's Charter on Fundamental Rights, Wathelet called it incumbent upon member states to offer gay couples, "like heterosexuals, the opportunity of having their union recognized in law and protected by the courts". The case was punted to Luxembourg in 2016 when Romania's constitutional court sought clarification on the underlying human-rights issues.
His advice is not binding, but the BBC reports ECJ judges generally followed the advocate's lead.
The couple challenged the decision, saying it was discriminatory on the grounds of sexual orientation.
Sophie in 't Veld, vice president of the Liberal group in the European Parliament and vice president of the European Parliament's LGBTI Intergroup, said: "This is fantastic news and a landmark opinion for rainbow families". Combined, they could soon require each of the EU's 28 member nations to extend the rights and benefits of marriage equality to LGBTQ people across Europe - European Union gay marriage right would be enormous deal affecting untold hundreds of thousands of people.
"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.
They turned down the request because Romanian law prohibits same-sex marriages and does not recognize such marriages that have taken place overseas.
"The fact that Mr. Hamilton did not live continuously with Mr. Coman in [Brussels] does not seem to me to be capable of rendering their relationship ineffective", he wrote.
Same-sex marriage is recognized in at least a dozen European Union countries-including Germany and Malta, which passed legislation a year ago.
Bulgaria, Latvia, Lithuania, Poland, Romania and Slovakia have no legislation on the matter - while Austria, Croatia, Cyprus, Greece, Hungary, Malta, Slovenia, the Czech Republic and Estonia all allow some form of civil partnerships.