An oath of celibacy is a promise that Deltans were obligated to swear to upon service in Starfleet, stating that they would not take advantage of any of their non-Deltan crewmates. This is necessary because Deltans are a highly sexualized species and view Humans as immature in this regard. Ilia filed her oath of celibacy before commencing her duties aboard the Enterprise. (Star Trek: The Motion Picture)