Freud on the Troilistic Enterprise

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.