Your thought process here is completely backwards. NASA's goal with the space missions was to get people into space and back to earth. This was not easy and required extremely capable individuals to carry out this mission. Had NASA wanted to ensure that both men and women were sent into space in the name of equality they would have had to delay the mission for several decades. This was because of the social conditions in the United States and you can harp on how terrible that was all you want, but that was a reality that had to be dealt with at the time. In the 50s you had a woman here or there who stood out at something or the other, but I sincerely doubt they could have found one who both met all the qualifications required of the program and also wanted to participate. This was only thirty years after women had acquired the right to vote. There weren't many (any) female military trained pilots who were also accomplished engineers and were in near perfect physical condition. Not even Jerrie Cobb. It's not because women lacked the potential, it's because society was not yet structured in such a way that they could realize that potential.
You can call 1950s American society sexist and you'd be right. Of course, no one would care and there's nothing controversial about that statement. Calling NASA sexist for existing in the 1950s is just dumb.