SCOTUS has affirmed that is not impermissible gender discrimination for US statutes to make the foreign-born children of US citizen mothers statutory citizens at birth while making the foreign-born children of US citizen fathers, such as the children of US servicemen conceived while abroad, jump through hoops following the birth, such as requiring proof of blood relationship or acknowledgment of paternity.
MILLER v. ALBRIGHT, SCOTUS 1998
http://www.law.cornell.edu/supct/html/96-1060.ZS.html
Per Justice Stevens writing for the majority:
(b) The §1409(a)(4) rule applicable to each class of out-of-wedlock children born abroad is eminently reasonable and justified by important Government interests: ensuring reliable proof that a person born out of wedlock who claims citizenship by birth actually shares a blood relationship with an American citizen; encouraging the development of a healthy relationship between the citizen parent and the child while the child is a minor; and fostering ties between the child and the United States. Male and female parents of foreign-born, out-of-wedlock children are differently situated in several pertinent respects. The child’s blood relationship to its birth mother is immediately obvious and is typically established by hospital records and birth certificates, but the relationship to the unmarried father may often be undisclosed and unrecorded in any contemporary public record. Similarly, the child’s birth mother certainly knows of the child’s existence and typically will have immediate custody, whereas, due to the normal interval of nine months between conception and birth, an unmarried father may not even know that his child exists, and the child may not know the father’s identity. Section 1409(a)(4)’s requirement–that children born out of wedlock to citizen fathers obtain formal proof of paternity by age 18, either through legitimation, written acknowledgment by the father under oath, or adjudication by a competent court–is well tailored to address these concerns. The conclusion that Congress may require an affirmative act by unmarried fathers and their children, but not mothers and their children, is directly supported by Lehr v. Robertson, 463 U.S. 248. Pp. 11—20.
(c) The argument that §1409(a)(4) is unconstitutional because it is a stereotypical “gender-based classification” must be rejected. None of the governmental interests underlying §1409(a)(4) can be fairly characterized as an accidental byproduct of a traditional way of thinking about the members of either sex. The biological differences between single men and single women provide a relevant basis for differing rules governing their ability to confer citizenship on children born out of wedlock in foreign lands, and an impartial analysis of those differences rebuts the strong presumption that gender-based legal distinctions are suspect. Pp. 20—24.
This woman is still today required to file as an alien resident. Your premise does not appear to jive with the facts nor does it appear to hold water. Otherwise ICE/INS would be following ex post facto from SCOTUS but in this woman`s case, their MANDATORY documented registration of an resident alien born of an American father and a Mexican mother in Mexico hasn`t changed position in 50 years. She married an American citizen, has 2 children and still has to register every year as a resident alien. I kid you not.