Women are a distraction. In a perfect world men and women would lose their sex drive when they went to war. This however, isn't a perfect world. Combat causes enormous stress as it is. The military doesn't need the additional headaches caused by co-ed troops.
Mating is difficult enough for civilians in peace time. Mating during combat just adds additional and unnecessary stresses to an already stressful situation.
OK, so you boot all your women. You're faced with replacing about 20% of your headcount. Where you going to get it from?