The UPCI's official explanation of why women should not wear pants:
"In addition, we should avoid ... slacks on women because they immodestly reveal the feminine contours of upper leg, thigh, and hip."
http://www.upci.org/doctrine/modesty.asp
This has always seemed chauvinistic to me, because slacks also reveal the male contours of upper leg, thigh, and hip...and sometimes other contours as well, if you know what I mean. Yet pants are acceptable for men to wear, but not for women.
Think about it.