U.S. District Judge Terence Kern said the ban, approved by voters in November 2004, violated the constitutional principle of equal treatment under the law.
The voter-approved ban said, "Marriage in this state shall consist only of the union of one man and one woman. Neither this Constitution nor any other provision of law shall be construed to require that marital status or the legal incidents thereof be conferred upon unmarried couples or groups."
It added, "A marriage between persons of the same gender performed in another state shall not be recognized as valid and binding in this state as of the date of the marriage."
But Kern cited U.S. Supreme Court rulings that struck down an anti-gay Colorado amendment and struck down the section of the Defense of Marriage Act that denied federal consideration of legally married same-sex couples.
"Against this backdrop," Kern said, "the court's task is to determine whether Part A of the Oklahoma Constitutional Amendment deprives a class of Oklahoma citizens -- namely, same-sex couples desiring an Oklahoma marriage license -- of equal protection of the law. Applying deferential rationality review, the court searched for a rational link between exclusion of this class from civil marriage and promotion of a legitimate governmental objective. Finding none, the court's rationality review reveals Part A as an arbitrary, irrational exclusion of just one class of Oklahoma citizens from a governmental benefit."
Kern's ruling came in the case of two same-sex couples who filed suit in 2004.
In issuing a stay, the judge noted the U.S. Supreme Court has stayed a judge's ruling permitting same-sex marriage in Utah, pending the outcome of an appeal.