Some issues to consider when it comes to allowing women in the infantry center around our current experience in war. While the United States and its culture has made marked progress in recent years in terms of womens' equality in the workplace, the military is a very traditional and conservative institution, and changes much more slowly than the rest of the country.
Part of that tradition stems from our own history in gender roles, where men are taught to feel it is their duty to protect the women and children of society, up to and including going to war. As outdated as that idea is, it is still engrained in our history and culture.
A primary issue both morally and ethically is the capture of female soldeirs on the battlefield. To employ female soldiers into combat situations or on the front lines is to do so with the knowledge that they may be captured. While male POW's have in the past been tortured and abused, very few are known to have suffered sexual abuse. At least three women captured during the first and second Iraq Wars have testified to such abuse after they were liberated.
Modern technology renders obsolete many of the older ideas about women's ability to fight on equal par with male soldiers, although there are certainly still duties and missions which call for brute strength and endurance, and not all women would be able to meet such challenges.
The decision of allowing or not allowing women to be infantry has very little to do with cardinal virtues or deadly sins. It is purely a matter of being more effective in fighting and winning a war.
Women are equal to men in most of the respect. But when it comes to fighting the a battle on the front as an infantry soldier, they definitely have the disadvantage of lower physical strength and size. Because of this women will be more effective contributor to the total war effort in areas where physical strength and power is less important.
This one consideration of size and strength is enough to justify exclusion of women from infantry. I see no moral or ethical issues involved in it.