Both characters are naive. Paul is a young private in Vietnam, and when the story begins, he has just witnessed Billy--a fellow soldier--die of fright. Throughout the story, Paul tries to process what he has seen and also how scared he is. He desperately wants to please his dad and be a hero, but his fear makes him feel like a coward and weakling.
Like Paul, Marilyn is young in age and in maturity. She thinks that her choice to board the spacecraft will get her a trip to see her brother; she doesn't realize the horrible ramifications of having to be "expelled" into space because the craft cannot hold two people. As she talks with Barton, the pilot, Marilyn's innocence wanes, and she realizes that life is not about simple choices. Additionally, she--like Paul--loves her parents and does not want to displease them or her brother.
In essence, both stories advance traditional "loss of innocence" themes.