Two couples, Mary Bishop and Sharon Baldwin and Gay Phillips and Susan Barton, filed their case challenging Oklahoma's ban in November 2004.
Kern referred to two previous Supreme Court decisions in his ruling. One, in 1996, ruled that Colorado could not pass laws taking away legal protections for gays, while the other, last year, ruled the federal government must recognize same-sex marriages that are legal in the states where they were performed.
"There is no precise legal label for what has occurred in Supreme Court jurisprudence beginning with Romer in 1996 and culminating in Windsor in 2013, but this Court knows a rhetorical shift when it sees one," he wrote.
Gay rights advocates hailed the judge's decision.
James Esseks, who directs the ACLU Lesbian Gay Bisexual Transgender & AIDS Project, said in an interview that the decision is "a reflection of how far we've moved on this issue."
"This is not an issue for the coasts, this is not an issue just for the liberals," he said. "This is increasingly America's understanding of what marriage is, and fairness in marriage laws looks like."
Human Rights Campaign president Chad Griffin in a statement said that Kern, who was appointed by President Clinton, "has come to the conclusion that so many have before him – that the fundamental equality of lesbian and gay couples is guaranteed by the United States Constitution. With last year’s historic decisions at the Supreme Court guiding the way, it is clear that we are on a path to full and equal citizenship for all lesbian, gay, bisexual and transgender Americans.”
However Brian S. Brown, president of the National Organization for Marriage, said in a statement the Oklahoma decision "is the latest in a string of examples of the dangers posed to state marriage laws when the avenue of debate is the federal court system."
To address these challenges, he added, the U.S. should pass a constitutional amendment "to protect marriage as the union of one man and one woman."
Six months ago the Supreme Court also let stand a lower court decision overturning California's ban on gay marriage.
There are now more than 40 cases pending in state and federal courts that raise the question of whether banning same-sex marriages is constitutional, Esseks said, adding that he expects the issue to return to the nation's highest court "within a couple of years."
Oklahoma's case falls under the jurisdiction of the U.S. Court of Appeals for the Tenth Circuit, which has already set an expedited hearing for the Utah gay marriage case.
Robert Barnes contributed to this report.