The Supreme Court has ruled that 8 USC 1409, which makes it more difficult for unwed US fathers to transmit citizenship to children born abroad than for unwed mothers, violates the guarantee of equal protection before the laws. "While the equal protection infirmity in retaining a longer physical-presence requirement for unwed fathers than for unwed mothers is clear, this Court is not equipped to grant the relief Morales-Santana seeks, i.e., extending to his father (and, derivatively, to him) the benefit of the one-year physical-presence term §1409(c) reserves for unwed mothers." Instead, the Court must "extend[] the general rule of longer physical-presence requirements to cover the previously favored group." "Going forward, Congress may address the issue and settle on a uniform prescription that neither favors nor disadvantages any person on the basis of gender. In the interim, as the Government suggests, §1401(a)(7)’s now-five year requirement should apply, prospectively, to children born to unwed U. S.-citizen mothers."
The full text of Sessions v. Morales-Santana:
https://www.supremecourt.gov/opinions/16pdf/15-1191_2a34.pdf