SCOTUS rejected hearing a case involving the draft and gender because there are literally only two constitutional outcomes of deciding such a case:
1. The draft is declared unconstitutional, weakening the US' ability to respond in case of a manpower shortage.
2. Females are forced to register with the draft, causing chaos as tens of millions of people are forced to register in order to vote, and as millions of people protest the draft since now it isn't just dudes potentially dying.