Firstly, there were a lot of stories written where fan authors attempted to introduce new female leads into the Star Trek universe. Unfortunately, as these stories tended to be formulaic, predictable, and lacking in 'believable' female heroines, they were swiftly dismissed by the fan community as a whole. A parody written by Paula Smith, "A Trekkie's Tale", was published in the zine "Menagerie" in 1974, and led to the coining of the term "Mary Sue" for any story with a predictable superheroine. Clicking on the title will let you read the story in its entirety - it's not very long.

So, if Lieutenant Uhura and Nurse Chapel were ultimately unsatisfying due to their subordinate roles on the ship, and new female leads proved unacceptable as a rule, where did these women have left to turn in their desire to see a relationship that was truly between equals?

"While Kirk loves many women in the course of the series, he loves none so dearly as he loves Spock; Kirk consistently renounces romantic ties that might interfere with his professional duties, while he has just as persistently been prepared to disobey orders and put his job at risk to protect his 'friend'." (Jenkins 203)