It's also a basic human instinct....Most guys would feel an overwhelming urge to protect the female......Goes back to the good ole days of protecting the species and such I guess, lol
I think you need to consider the effect one woman would have on a platoon. Infantry living conditions are cramped to say the least and toilet arrangements are disgusting. A woman in that situation would upset the status quo as I don't know many men who like to go to the loo 2 feet from a woman.
It's not a sexist issue, it's a morale issue. Women would bring down the morale and make everyone uncomfortable.
Not to mention the entire male-female interaction. Suppose she likes soldier 'A', but not soldier 'B'. Now we have internal conflict because soldier 'B' is trying to get to know her. Please...don't say this doesn't happen, because it does. I've seen it. It can wreak havoc until things settle out, and that takes time. One thing you don't have a lot of between firefights.