It's an oversight, plain and simple. The generic (over-accented) nord female voice has a full set of Legion-aligned barks, for both in camps and as guards.
Also, if the Legion is "sixist", then why are the (initially) pro-Empire town guard ranks exclusively male too?
To me, the Empire was painted as the more egalitarian of the two sides of this conflict (cultural posturing aside... which doesn't compare at all to the outright racism of the Stormcloaks). Which is why there being no women in the Legion ranks was kinda conspicuous.
If you're on PC, http://www.gamesas.com/topic/1351761-legionettes/
