Yes, the military is made to fight, the most important thing it does. But one must keep in mind that the military must do other things, like march and set up camp. Even in a support role most women and many fat fuckers can't keep up on the march or do the tasks that are assigned to them, worse, and most importantly, they become non combat casualities that weigh the system down.
On the march, fatso and the old peasant who can't keep up slow down the march and make logistics harder. When the 60 year old chronic smoker shows up he's winded before he even starts to march anywhere. Next thing you know you've got bellyachers and people with normal movement injuries from above normal activity in these people, ranging from sprains and breaks to exhaustion and heart attacks. The infrastructure has to deal with everything from increased minor injuries that decrease capacity and capability to increases in severe issues. The military can deal with one strong young man with a heart defect that cleared his screenings and dies in the field; it's not so great when a whole pile of asthmatics and poor shape individuals start to have these issues at rates sometimes tens of times higher by rate. Women have high rates of broke breaks and hip fractures from heavy marching, they are built to have babies not march isn't some sexist mantra its scientific truth.
We need reliable men in the field, not broken down people in the back lines being treated for minor nonsense. Yes, many of these people might be useful in defense scenarios, they might be useful to a guerilla group that operates locally, but on the march general army many of these people are rejected for good reason. A broken down truck fails to deliver on time, clogs the road. Broken down men are the same, and the longer and harder they must push the more likely they are to break down and get in the way.
Anyone who knows me would say "But you say the old men were the best at fighting" and I'd say yes, the old warriors were always the best. But, that's old warriors who fought and won. Old peasants neither can fight or keep up, they are just get in the way of the general army. Almost all young peasants can't fight, but if they are drafted or volunteered they can at least march and take camp life, which used to kill many through attrition, and is still a major consideration, and provide something of value while not breaking, failing, obstructing.