“Mr. Flores-Villar argued that the differing treatments violated equal protection principles. The Supreme Court has said that sex-based classifications are permissible only if they serve important governmental goals and are substantially related to achieving those goals. In 2001, the Supreme Court upheld a law that imposed differing requirements in a similar situation. In that case, Nguyen v. Immigration and Naturalization Service, a closely divided court said that American fathers of children born out of wedlock abroad had to get a court order establishing paternity or swear to it under oath for their children to obtain American citizenship. American mothers were not subject to that requirement.”