The European Court of Justice (ECJ) will still need to rule on the case - brought by a man whose American husband was refused residence in his home country of Romania - which could expand legal recognition of same-sex marriages across Europe.
The opinion was released as the ECJ hears the case of Romanian Adrian Coman, who tied the knot with his American partner, Claibourn Robert Hamilton, in Belgium in 2010.
Romania has laws that prohibit the marriage of same sex couples, and doesn't recognise them at all in the legal system - resulting in the Romanian nationals spouse being unable to take up residence as the spouse of an European Union citizen.
Belgian advocate general Melchior Wathelet wrote today that European Union countries must recognise same-sex spouses in the same ways that they do straight spouses.
EU law [materials] permits the spouse of an EU citizen to obtain a residence permit for the EU member state where the spouse resides.
Concluding, he said: "In that context, the advocate general considers 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. the term "marriage means a union between two persons of the opposite sex" can no longer be followed".
Advocate General Melchior Wathelet said the term "spouse" should include partners of the same sex when it came to the freedom of residence of European Union citizens and their family members.
Advising the European Court of Justice on the case, Advocate General Melchior Wathelet said Thursday balanced Romania's interest in protecting "the traditional family" against its obligation not to discriminate on grounds of sexual orientation.
The judge added that "the objective of protecting the traditional family can not justify discrimination on grounds of sexual orientation" and that "the concept of "spouse" within the meaning of the directive also includes spouses of the same sex".
Romania's constitutional court referred the matter to the ECJ.
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. 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 very big deal affecting untold hundreds of thousands of people.
"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.