Polymorphous perversity is a psychoanalytic term for the human ability to gain sexual gratification outside socially normative sexual behaviors. Sigmund Freud used this term to describe the normal sexual disposition of humans from infancy to about age five.
Freud theorized that humans are born with unfocused sexual libidinal drives, deriving sexual pleasure from any part of the body. The objects and modes of sexual satisfaction are multifarious, directed at every object that might provide pleasure. Polymorphous perverse sexuality continues from infancy through about age five, progressing through three distinct developmental stages: the oral stage, anal stage, and phallic stage. Only in subsequent developmental stages do children learn to constrain sexual drives to socially accepted norms, culminating in adult heterosexual behavior focused on the genitals and reproduction.
Freud taught that during this stage of undifferentiated impulse for sexual pleasure, incestuous and bisexual urges are normal. Lacking knowledge that certain modes of gratification are forbidden, the polymorphously perverse child seeks sexual gratification wherever it occurs. In the earliest phase, the oral phase, the child forms a libidinal bond with the mother via sexual pleasure gained from sucking the breast.
For Freud, "perversion" is a non-judgmental term. He used it to designate behavior outside socially acceptable norms.
See: Freud, Sigmund. The Standard Edition of the Complete Psychological Works of Sigmund Freud. Trans. James Strachey. 24 vols. London: Hogarth, 1953-74.