The Space Race was not a game. Nor was it something in the far distant future. It was deadly serious, immediate, and with far-reaching geopolitical consequences in a world where the threat of global nuclear annihilation and totalitarian communism was very real, and where much of the world was made up of non-aligned, newly independent nations trying to decide whether to ally with the West or with Moscow. Demonstrating technological superiority in space was an important tool in swaying people & nations to side with us.
The stakes were high and the list of unknowns stretched beyond the horizon. Some of the early medical examinations and experiments foisted upon astronaut candidates were truly bizarre, because nobody really knew anything about space, or what to expect, or how zero gravity or radiation would affect the human body. So they essentially made stuff up and did everything they could think of to the astronaut candidates, including lots of cameras and metal probes in places where the sun don't shine. Imagine the awkwardness of including women in such invasive and seemingly pointless examinations. Imagine the self-limiting and self-censoring effect that would have had on the early doctors and program architects who were trying to navigate their way through completely unknown territory, trying to get astronauts prepared for space.
Virtually all the astronauts were military, and served as combat fighter pilots or test pilots or both. Why? because they:
1) understood mortal danger
2) understood chain of command
3) were accustomed to long, strenuous, invasive, harsh training regimens
NASA wasn't selecting from a pool of "men" but from the pool of "fighter/test pilot". Women were excluded not primarily (or only) because they were women but because they were CIVILIAN. Why? Because women generally were not part of military combat. Why? Because our culture wasn't (and still isn't) ok with shipping millions of women off to fight, kill, bleed, and die. Men, yes. Always.
Men have always been the expendable sex to be shipped off to kill and die. We dress it up in shiny ways to make it more palatable, with stuff about "honor" "duty" etc. but the brute fact is men are expendable, so they get to die. One of the consequences of this is that occasionally, this actually leads to a benefit and privilege, like prestige assignments such as the astronaut corps. When your mortal enemy suddenly has a commanding and existentially threatening lead in space (If they can launch satellites into space, they can launch nuclear warheads into space) and you need to quickly create a human space program in response, you draw from the most appropriate existing talent pool: fighter pilots & test pilots.
Viewed in its proper historical context, crying "boo sexism! men bad!" seems utterly myopic & narcissistic, and emblematic of a sheltered 21st century perspective that wants to re-frame Mercury, Gemini, and Apollo as some kind of entitlement program that women were unfairly and deliberately excluded from, rather than something the U.S. desperately needed to succeed. The space program didn't exist to make astronauts feel good, fulfilled, and self-actualized. Astronauts were selected, trained, and used as guinea pigs so that the space program could succeed. Before asking "why weren't women *allowed* to be in the space program?", one must first ask "why weren't millions of women *allowed* to be drafted against their will to fight, kill, bleed and die, and to be used as guinea pigs in dangerous programs since time immemorial?"
