Well, there's Lester's dialogue, plus the reality that we never saw a woman in command of a starship (or, after "The Cage," even close to commanding one).
How many other (UFP) starship commanders, past and present, did we actually see
in the course of TOS?
Pike, Decker, Wesley, Tracy, Stone. A few other "maybes".
There were references to "Captain Harris" of USS Excalibur, and to the Captain (unnamed) of USS Intrepid, but no clues as to gender.
I totally agree that there was a bias against showing women in command during TOS's run. Trekverse-wise, the fact that we never see them does not prove their non-existance. There are several ethnic groups (and alien races) that we never saw (or heard of) as being TOS starship captains either, but I presume that they existed.