As far as I'm concerned TOS never spelled out that women couldn't command so I choose to accept that they could because it's not contradicted otherwise.
We're talking past each other, discussing two separate issues. Of course I agree completely, as I have already said, that it's best to interpret TOS's conjectural universe as one that allows female captains. That is not even in dispute as far as I'm concerned. But I'm talking about a different issue, which is not about the overall hypothetical universe of the 2260s Federation, but about the creative process and intent behind the writing of specific individual scripts for the 1960s television series named Star Trek
. We can certainly, gladly disregard the sexist authorial intentions behind "Turnabout Intruder," but there is no doubt in my mind that the intentions were there -- not in the series as a whole, but definitely in that particular teleplay.
On that point then I want to see a memo or firsthand account or some other documentation that establishes the TOS staff saying, "We won't show any women in command positions." I want to see someone's story outline proposing a female command rank character and then having it shot down.
Basically I want to know if the idea was even pitched or it just never occurred to anyone including D.C. Fontana. Seems strange to me because it seems exactly the sort of thing Fontana would have liked to see happen.