A U.S. federal judge has upheld Puerto Rico's ban on same-sex marriage and ruled that the U.S. Supreme Court's ruling last year that legalised gay marriage does not apply to Puerto Rico since it is an "unincorporated territory" and not a U.S. state.

In his decision, U.S. District Court Judge Juan Perez-Gimenez wrote, "One might be tempted to assume that the constant reference made to the 'States' in Obergefell includes the Commonwealth of Puerto Rico. Yet, it is not the role of this court to venture into such an interpretation."

He said, "For the foregoing reasons, the court concludes that the fundamental right to marry, as recognized by the Supreme Court in Obergefell, has not been incorporated to the juridical reality of Puerto Rico."

"Thus, the court declines to hold that the marriage ban codified in Article 68 of the Civil Code violates the Due Process and the Equal Protection Clauses of the Fourteenth Amendment by denying same-sex couples in Puerto Rico the right to marry or to have marriages validly performed in another jurisdiction given full recognition," the judge ruled.

The ruling stemmed from a lawsuit filed two years ago by three same-sex couples who got married in Massachusetts, New York and Canada but wanted their marriage to be recognised in Puerto Rico; two same-sex couples who wanted to marry in the island; and LGBT advocacy group Puerto Rico Para Tod@s.

Perez-Gimenez previously upheld the marriage ban in October 2014, according to Buzzfeed News.

After the U.S. Supreme Court legalised same-sex marriage in June last year, the First Circuit Court of Appeals vacated the district court's decision and remanded the case back to Perez-Gimenez, saying that "we agree with the parties' joint position that the ban is unconstitutional."

But the judge still upheld the marriage ban.

He said if same-sex marriage is to be applied in Puerto Rico, there should be a ruling by the U.S. Supreme Court and Supreme Court of Puerto Rico, "incorporation through legislation enacted by Congress, in the exercise of the powers conferred by the Territorial Clause" and "by virtue of any act or statute adopted by the Puerto Rico Legislature that amends or repeals Article 68."