In my humble, humble opinion, it boils down to a simple risk vs. reward scenario.
Reward: The numerical insignificance of females potentially capable of infantry service severely limits the reward inherent to such a policy.
Risk: Introducing the added element of human sexuality into combat units.
The choice is obvious.
"The great uncertainty of all data in war is a peculiar difficulty, because all action must, to a certain extent, be planned in a mere twilight, which in addition not infrequently — like the effect of a fog or moonshine — gives to things exaggerated dimensions and unnatural appearance."
-- Carl von Clausewitz
Children and the elderly die all the time in wars. That's why we should try to avoid wars as much as possible.After all, there are plenty of women in war zones.
If men's behavior is too "un-nice" to allow female soldiers to share the battlefield with them, then is it really appropriate to allow them around female civilians and their children and elderly?