sexuality, ordinarily thought of as a unity, was decomposed into a plurality of separate drives; and since it was tacitly assumed that sexuality originates in the genitals, Freud arrived at the conception of ‘erogenous zones,’ by which he meant the mouth, skin, anus, etc.
Jung traces precisely how Freud’s observation of infantile polymorphous-perverse sexuality forced the theoretical decomposition of unitary sexuality into component drives organized around erogenous zones.
, Collected Works Volume 1: Psychiatric Studies, 1902thesis