At one point Captain Norton ponders how his crew frequently go off and have discreet sex (and he can't because of his position), but he has so far avoided the problem of a shipboard pregnancy--although, it is noted, it is a risk. Later, Clarke's narrative explains that all spacemen are sterilized so that radiation exposure won't lead to mutant babies. If they're all sterilized (including Norton himself), why are there unwanted pregnancies aboard spaceships?