Military science fiction is politically correct; if you read much in the sub-genre you've seen mixed-gender combat units. Except for, basically, the Starfist and Starfist: Force Recon series written by Dan Cragg and me. Dan and I chose not to use mixed-gender units because, frankly, they don't work in reality, and we try to be as realistic as possible in our books.
I need to define my terms before I continue.
When I say combat units, I'm not talking about air forces or navies, sea or space.
I was an infantryman. To me, combat units are ground combat, primarily infantry.
There are several series about space navies, notably David Weber's Honor Harrington and Jack Campbell's (John G. Hemry) Lost Fleet.
On a panel on which John Hemry and I were together at this year's Baltimore Science Fiction Convention, I remarked that I'd made a career of writing novels about young men at war. John cut in with, "I write about young men and women at war." I replied, "You write about the navy, I write about Marines," and continued with what I'd been saying.
But that brief exchange stuck with me, and I need to amplify on it. So here goes, a comparison between shipboard life and life on the ground.
Let's start by looking at living conditions aboard a warship:
A warship has separate sleeping quarters for men and women. The racks (navy for bed) might be narrow and stacked close together, but they have mattresses and sheets which are regularly laundered.
A warship has separate heads (navy for bathroom) for men and women.
A warship has separate shower facilities for men and women, and everybody is expected to shower frequently.
Those last three items grant a modicum of privacy to men and women aboard warships.
A warship has scuttlebutts (navy for water fountain) all over the place for when you're thirsty; the scuttlebutts may even have cold water.
On a warship, you get to sit on a chair at a table probably three times a day to eat a hot meal that was prepared by somebody else,. So what if you have to bus your own table, somebody else washes the dishes.
You get to wear clean underwear, socks, and uniform every day.
A warship has a library, television, radio, the internet and email, bowling alleys, and shows movies for the crew.
Now compare that with the living conditions of an infantryman in the field:
Sleeping quarters? The dirt, sand, mud, whatever that you've been slogging through. Okay, you've got a sleeping bag that you lugged along. Personally, unless I'm someplace very cold, I'd rather leave that weight behind.
Heads? The nearest bush, thank you. And depending on circumstances, you might go in front of that bush instead of behind it.
Showers? If you're out long enough, you might encounter a stream, river, pond, lake, or other body of water big enough to bathe in. If your commander thinks the area is secure enough to allow a few people at a time to strip down and take a quick bath—under guard.
Privacy? Ain't no such thing.
Scuttlebutts? You carry your own water, fella. Back in the day, we carried two one-quart canteens. Today the troops have "camelbacks": which, I think, hold two gallons. Cold water? Only if it's freezing outside. I must admit, the Marines and army are much better today about resupplying troops in the field with fresh water, but. . . you still might find yourself in a situation where you have to drink from a stream, pond, well, whathaveyou, and risk getting infected from whatever parasites or pathogens might be in the water. So you purify the water with iodine or halizone tablets, both of which give the water a flavor that many find distasteful.
No chairs, no table to eat at. Nobody else cooked your meal. You carry your rations on your back, and they're hot only if you have the opportunity to heat them yourself. Forget bussing the table, you probably have to lug your waste out yourself.
Clean clothes? If you're smart, you'll carry an extra pair of socks or two so that you can change into dry ones daily. Otherwise, you wear the same clothes until you come in out of the field. And if you're going to be out more than a few days, don't bother with underwear because they can get moldy and cause crotch rot. Seriously.
Long hours? They don't stop until you come back in. Lots of luck on getting any kind of decent sleep.
No library, television, radio, internet, email, bowling alleys, or movies. Maybe some snail mail.
Here's one of the biggest differences between shipboard life and humping through the toolies:
Okay, the work hours in the navy can be long, and the work is sometimes strenuous, but if you have to pick up something heavy, you probably don't have to haul it terribly far.
In the infantry, on the other hand, you don't have to pick up something heavy and carry it a relatively short distance. Nope. The infantryman's basic combat load that he carries every step he takes, is 120 pounds. That includes weapons, ammunition, water, food, first aid kit, night vision goggles, body-armor, clothing, sleeping bag, and a whole load of other necessities (some of which old-timers like me think would be better left behind). On top of the basic load, you might have to carry a couple of mortar rounds, a twenty-pound satchel charge, a thousand-round box of machine gun ammunition, or other goodies.
Oh yeah, I forgot. That 120 pound basic load? That's for riflemen. Machine gunners, mortarmen, and radio operators have heavier basic loads.
Most men in the military weigh in the 160-180 pound range, women 120-135. That has the average man carrying 70%, give or take, of his body weight. Using the same basic load, the average woman would be carrying roughly 90% to 100% of her body weight. In each case, it's plus any extras (mortar rounds, machinegun ammunition, satchel charge, etc) the infantryman might be humping.
The infantryman not only carries that weight, he's expected to be able to fight at an instant's notice—fight and win.
My combat weight was about 135 pounds. But we didn't hump 120 pounds in our basic load. I've calculated that my basic load was 70-75 pounds. Which sometimes was increased by twenty pounds of satchel charge or whathaveyou. Roughly 50% to 70% of my body weight. Take my word for it, humping all that weight in tropical heat and humidity was exhausting. I remember a particular combat operation. My company was going through an area of brush that was mostly lower than a man's height when we came under fire. I was greatly relieved to be able to get down, under a bush that gave me some relief from the tropical sun. Yet my load was far less than today's combat load.
There are some fairly valid reasons for today's greater weight: Today the infantryman carries sixteen pounds of water instead of the four pounds we carried. Night vision goggles, a very worthwhile addition to the infantryman's weaponry, add a few pounds. I don't remember how much the flack jacket weighed; most of us didn't bother wearing it because of its weight and the way it kept heat inside. We thought the chance of getting shot was less than the chance of being felled by heat stroke brought on by wearing the flack jacket. Today, all infantrymen wear body armor in the field. I have mixed feelings about the body armor, I'd like to see some statistics on heat exhaustion and heat stroke in Iraq and Afghanistan. The sleeping bag is a nice creature comfort, but not if it makes you fall over from the heat, not if it leaves you less capable when you have to fight.
If carrying 70% or more of his body weight all the time is stressful for a man, how much more stressful would it be for a woman carrying 90% or more of her body weight?
Here, I'm not dealing with issues of hormones, biological imperatives, or the way young men and young women react to each other, just the weight that infantrymen carry. Looking realistically at that point alone, mixed gender infantry units are unrealistic.
No, don't tell me there's an easy solution, simply provide vehicles to carry much of the weight. Not only would the vehicles add to possibly already-stressed logistics, but infantrymen commonly go places vehicles—and even pack mules—can't.