NEW ORLEANS — A federal judge here upheld the state’s ban on same-sex marriage on Wednesday, going against what had been a unanimous trend of federal court decisions striking down such bans since the Supreme Court ruled on the matter last year.

In his ruling, Judge Martin L. C. Feldman of Federal District Court said that the regulation of marriage was left up to the states and the democratic process; that no fundamental right was being violated by the ban; and that Louisiana had a “legitimate interest ... whether obsolete in the opinion of some, or not, in the opinion of others ... in linking children to an intact family formed by their two biological parents.”

That this ruling ran counter to a wave of other federal decisions across the country in recent months was immediately noted by opponents of the ban.

“We always anticipated that it would be a difficult challenge,” said J. Dalton Courson, a lawyer for the plaintiffs, adding that the ruling would be appealed to the United States Court of Appeals for the Fifth Circuit. “We certainly are disappointed, considering the string of rulings in favor of same-sex marriage.”