It's "almost" non-existent. Notice how just three examples:
1. None of the Emperors have been women.
Oh you are so freaking wrong! Go read up on the histories, there have been plenty of female emperors. [censored], a dark elf was an empress once, and lead Tamriel into a [censored] golden age during her reign.
2. Uriel Septim had no daughters.
So...Anyone who doesn't produce a daughter is now sixist? That's a little harsh.
3. Most of the leaders in Oblivion were male (and if you say "Mages Guild", remember who the archmage was).
Yeah, I guess that being the lady who was the head of the Fighters Guild doesn't count, or that there are plenty of high ranked members of the Thieves Guild, Fighters Guild, and Mages Guild. And ignore that the PC can be a leader of every organization as a female.
4. Many of the leader roles, with whom the player character interacted, were male.
Hrmm...either you ignored most of Morrowind (or never played it), or you are cherry picking too much.
Now before you bite off my virtual head,
Too late.
Also to note, you should take account of Morrowind too. There are plenty of female leaders and the like in nearly every organizations. The Imperial Guard may be the exception, but I never played that. And you should note that the PC, as a female, can also become a leader to all organization the PC joined, assuming the PC has the skill and attribute requirements (for MW at least). No one can become a head in DF, only a really high rank.