President Obama has criticized the law, but his Justice Department is defending it in court. The administration says Congress was entitled to preserve the status quo in federal law while states debated the marriage issue.

But Wilken said the 1996 law actually changed the status quo by "robbing states of the power to allow same-sex civil marriages that will be recognized under federal law."

She also rejected arguments that the law's sponsors put forth in 1996, that the legislation was necessary to promote procreation and preserve heterosexual marriage.