My understanding is that the death rate in each age cohort was about the same for males and females, controlling for the additional risk to pregnant women. Finding out exactly why so many otherwise healthy adults died in the Spanish Flu has been a subject of considerable interest for many years.

Of course, military service can be debilitating all by itself, and troop concentrations and movements are thought to have influenced the pattern of the epidemic, but I don't believe current thinking is that incidents to military service explain the unusual "W" shaped, instead of the usual "U" shaped, distribution of mortality. (a peak on the left for infants, a peak in the middle for adults, and a peak on the right for the elderly, where x = increasing age and y = increasing death rates)

Consider why there would be interest in that, especially if it were the case that it was preferentially debilitating or lethal to adult males. Not all research into influenza has been based on benign motives.

Jeff