Fortuitously, local animal species do not include mammals, in which sexual differences are easily discerned. Finally, the women do know about what they call "Monsters," non-human intelligent species that might also arrive in spacecraft. Therefore, when one man does arrive alone, he must prove that he is not a Monster.
He is put naked in a cage with a Maiden, a women who has not yet been fertilized, in front of a crowd and told to fertilize her. She gives him her permission so this would have been consensual sex with an adult!
Individuals and mores differ. Like many of us, he is unable to function in these circumstances. (In "Rammer" by Larry Niven, an inability to have sex in front of witnesses was interpreted as evidence that the viewpoint character would be able to handle the isolation and celibacy of a long solo interstellar journey at relativistic speeds.) My only comment here is that, although our hero's predicament is understandable and expectable, it is not inevitable. I have known men who would willingly and gladly prove themselves in these circumstances... But then Anderson's story would have been over very quickly, as Hamlet would end quickly if its hero could just do what he thinks he ought to do in the first place!
(Arnold Shwarzenegger's Hamlet rewrite: "You killed my father, Claudius! Big mistake!" Gun fire. Curtain.)