They aren't pressing for 'sex' to be defined in law, or for women to be drafted, or for the end of the draft but they do think that saying that only men should be drafted is sex discrimination.
''Ending sex-based registration would be a significant advancement in equal protection law for men, women, and LGBTQ people.''
www.aclu.org/cases/national-coalition-men-et-al-v-selective-service-system-et-al