There aren't female head coaches in the NFL because they have separate leagues for men and women. Even in the women's national level of sports though, female coaches make up about 50% of the total.
Females in the army have historically served roles that aid soldiers as opposed to being in combat (I.e.: nurses), but that doesn't mean there hasn't ever been a female in ground combat. In the Civil War, when a male soldier was shot a woman sometimes picked up his weapon and fought back. There have also been a few instances where women have impersonated men to get into infantry units.
There's not law that prevents women from fighting. The military basically can make it's own policies, and they decide the extent of women's roles in the military. So at any time, the higher ups can say the word and allow women to fight.
my mother's in the armyShould I join th army