As we may observe with children and soldiers, common activity is not excluded even in the excretory functions. The one great exception is provided by the sexual act, in which a third person is at best superfluous and in the extreme case is condemned to a state of painful expectancy.
Freud, Sigmund. Group Psychology and the Analysis of the Ego. Trans. and ed. James Strachey. New York: Norton, 1957. 54-55 n.3.