There is a distinct biological reason that women are not in primary combat roles. It is linked to the reason they also create logistical concerns.
#1. Field Latrines:
The proper digging of latrines in the field changes from male to female. A female soldier, because of the abundance of potential feminine products, requires what is known as a "trench latrine". These leave a larger mark for opfor to locate and use in tracking, in contrast with a "pit latrine" (used for males), which is much smaller, and leaves a less of a footprint.
#2. Biological scents:
Female soldiers, although certainly the type of motivated, calculated killer I want behind a .50 during certain periods (is that a pun?) of the month, unfortunately leave a strong trail of odor, be it perfume from pads/tampons, or biological excretion, that a tracking unit equipped with dogs, could easily pursue. This endangers operatives, and infantrymen alike during critical maneuvers. The Taliban for example, have been known to use tracking dogs to pursue field ops or patrolling squads.
#3. Logistics:
Packing kotex pads or tampons in an already excessive loadout is not my description of traveling light, specifically when the FOB or other base in the AO might not be seen again for 2-3 weeks.
#4. Airlifted supply drops:
Ok, so it falls under logistics, but it needs specific mention because it would be a huge problem. Personally, I want more dope, than soap and tampons. I know some women who are expressly for female 11B's are going to try and say "Wah wah, throw em on the pallet!", but every airload that goes out is a precise measurement on what can be carried and/or balanced in transport. Ask any loadmaster who just loaded up a CH-46 to throw in this box of tampons, and he is going to say, "You want me to ditch some water, or ammo?". He is being serious by the way.
#5. Medical Requirements:
It's already dicey in combatland, without having to also assure that an OBGYN is present and accounted for, in case a female soldier gets shot in the pelvis. Medically, this presents a can of worms for current combat support hospitals.