The 7th Circuit Court of Appeals on Thursday struck down gay marriage bans in Indiana and Wisconsin, affirming decisions by lower courts.
The three-judge panel ruled unanimously that the two states "have given us no reason to think they have a 'reasonable basis' for forbidding same-sex marriage."
Those earlier decisions are being appealed to the Supreme Court.
The panel was unconvinced by the states' argument that the government has an interest in promoting traditional marriage only to prevent births out of marriage, which sometimes lead to abandonment. Same-sex couples do not need marriage because they cannot produce children, the state had argued.
"Overlooked by this argument is that many of those abandoned children are adopted by homosexual couples, and those children would be better off both emotionally and economically if their adoptive parents were married," according to the opinion.
Judge Richard Posner, appointed by Ronald Reagan, wrote the 40-page opinion. The two other judges that affirmed were: Ann Claire Williams, appointed by former President Clinton, and David Hamilton, appointed by President Obama.
The panel did not order the decision to be stayed pending appeal, though the Supreme Court has put similar decisions on hold.
This story was updated at 4:15 p.m.