Women have been involved in the military since the beginning of the 20th century in auxiliary and medical roles, but their participation in other roles has rapidly risen since the early 1970s. As of July 2010, women were allowed in the U.S. military but were not allowed in direct combat positions like infantry and special forces. There are some disadvantages to having women in the U.S. military.
Genetically, women are not as physically strong as men. This is not a downside in command, support, and specialized positions like helicopter pilots. It is, however, a strong disadvantage in combat. Even in the 21st century, an infantryman has to carry a great deal of equipment, often weighing between 50 and 100 pounds.
The fact is that women have a harder time moving quickly with this much gear on their backs, and an equally hard time maintaining pace over a long period of time, regardless of how fit they are.
Because women are such a minority in the military, there are liability issues with assigning women to majority-male units. These are mostly to do with sexual harassment--the Army does not want a situation where a man is alone with a woman, as any potential sexual harassment case would devolve into one person's word against another's.
The Army has combated this by either assigning zero women to a unit, or assigning at least two--and then enforcing a rule that they cannot leave one another's side. This creates a situation where it is difficult to get things done efficiently, whether the women involved are military police, supply, or HR workers.
American Cultural Values
American cultural values are staunchly in favor of "protecting" women. Putting women in situations where they may find themselves captured and possibly sexually abused runs counter to those values. This is made more likely by two factors: one is the blurring of front lines in 21st century wars, which means that capture is more likely regardless of a woman's military occupational specialty. Furthermore, women are substantially more likely to be raped in captivity than men.
It is useful to note that, at various times in history, American cultural values also opposed African-Americans serving in the military because of their supposed limited mental capacity. The first African-American graduate of West Point was shunned during his entire four years there due to the color of his skin. America's cultural values have changed and the same may prove true with women in the military.