The ability to reclassify legal sex as male (M), female (F), and even X, is a core issue of trans and non-binary legal engagements. A wave of legal reform and debate has recently swept across the United States, resulting in a spectrum of laws and policies, from a complete ban on reclassification to the innovative framework of self-identification. This Article provides a comprehensive study of American sex reclassification law, exploring a paradigmatic shift in legal preconditions for recognition: from demanding proof of medically authorized “sex reassignment” toward protecting the individual right to “gender identity.” Investigating these shifts, this Article argues that even the most expansive reclassification framework fails to address the pervasive harm caused by the initial act of assigning sex at birth. The Article argues that the incongruence experienced by trans legal subjects is not between their body and identity but between their birth assigned sex and the gender expectations associated with it. Instead of merely seeking improved reclassification frameworks, the Article asks how sex classification impacts both trans and non-trans (cisgender) legal subjects. Linking the needs of trans and non-binary people to broader questions of sex in law, the Article lays the groundwork for envisioning the potential end of assigning sex at birth and the separation of the differentiated status M/F from the natal body.