Actually, the Empire does commission female officers.
In Star Wars: The Force Unleashed, a video game which is considered canon in the SW universe, a female Imperial Officer is assigned to be Starkiller's pilot and aide.
In the novel "Death Star", there is a female admiral who rose through the ranks on her own merits, much to the chagrin of other male officers, especially of equivalent rank. She is also the love interest of Grand Moff Wilhuff Tarkin.
So, the Empire does indeed have female officers.....the only visible one was in the Force Unleashed game. We've not seen them in any movies, but one did appear in a game that was considered canon.