THE LAW
Selective Service law as it's written now refers specifically to "male persons" in stating who must register and who would be drafted. For women to be required to register with Selective Service, Congress would have to amend the law.
The Department of Defense noted that America's prior drafts were used to supply adequate numbers of Army ground combat troops. Because women are excluded by policy from front line combat.
Now that combat roles in the military are opening up for women, change the law so it is written "male and female persons." Women have fought for years for equal rights, so it is about time they had them.