You can't afford to have a bunch of guys deciding they have to "protect" their female squadmate, or abandoning their post because they're to rescue someone they're risking a court martial to sleep with when not in combat. It's a question of whether the MEN, not the women, are good enough; the latter is easily ensured by uniform requirements for all soldiers. All the same concerns are present when homosexual soldiers are enlisted, with the added wrinkle that since they all share the same barracks the risk that some of them are sharing the same bunk increases, with the damage to discipline and morale that threatens.