I guess I make a distinction between "guard" on the one hand and "soldier" or "officer" on the other hand. When the OP uses the word "guard" I personally don't think of "soldier" or "officer." I think of the actual guards who stand near city gates, patrol streets and farms and arrest you after a crime.
If we expand the definition of guard to mean anybody who wears an Imperial uniform then, yes, I agree, there have been female Imperial guards. I'm going to have to go take another look at Falkreath and Solitude. Offhand, I don't remember seeing female Imperial guards. But then I wasn't paying any attention to the issue either.
