I know "The Right Stuff" (great film by the way!) is not a true documentary/docudrama and has segments of fictional portrayal, but the doctors and scientists were interested in astronauts sperm, blood, saliva, urine, fecal matter, sweat - pretty much anything exuded or expended from the human body. They were specifically interested (in the case of your question) in being able to analyze and compare the sperm count both pre and post-flight, and see how active the sperm were pre and post-flight.

You gotta keep in mind here that this film was chronicling the earliest days of space flight, where very, very little was known about the human body's exposure to a microgravity environment, or the effects/amounts of cosmic radiation at that altitude above the Earth's atmosphere. The scientist/doctors wanted to do every test they could, before and after flight, to study not only the direct effects on the human body during spaceflight, but also to conduct a comparative analysis between anything they could sample pre-flight and post-flight. Those pre and post-flight samples could be compared over time to see if there were any measurable effects caused by the journey above the atmosphere.