I wouldn't define Selective Service as a "privilege" benefitting men/boys. In fact, it's a weapon through which men/boys are pawns to maintain a patriarchal system.

Rather, it's based upon the traditionalist fallacy that only men are capable of serving in combat. The belief that women shouldn't be fighting in wars. Thus, architects of the patriarchy keep Selective Service as male-only in order to uphold those ideals -- and to keep men and women in separate roles based on their genitalia.