I believe when Roddenberry was selling the pilot, he wanted the #2 to Kirk to be a female (and cast Majel Barrett) but the studios would not have a woman second in command. Roddenberry came back with an alien (Spock) and even though he had green blood and was essentially an egghead, the studios agreed. There was zero chance a female could be a heartbeat away from the space presidency. That was over 50 years ago, and Star Trek has come a long way, baby.