Majel Barret was originally cast as the first officer in the pilot, but she had to give it up because the viewers wouldn't accept her role. (the brass claimed).
Gene Roddenberry was told by the NBC brass to get rid of Majel Barret becasue no one was impressed with her acting ability, also because Roddenberry - a married man - was placing his girlfriend in such a important role (third billing?), she simply didn't possess the acting credentials . Roddenberry was perfectly free to recast the role with another woman, he declinded.
This was the mid-sixties, womans lib was a cultural force, women were making more buying decission for the house holds, NBC
loved the idea of a female first officer, they just didn't want Majel Barret.

