From Sigmund Freud’s “Introduction to Psychoanalysis“:

“What we call “perverse” in the life of the adult, differs from the normal in the following respects: first, in disregard for the dividing line of species (the gulf between man and animal); second, being insensible to the conventional feeling of disgust; third, the incest-limitation (being prohibited from seeking sexual satisfaction with near blood-relations); fourth, homosexuality, and fifth, transferring the role of the genitals to other organs and other parts of the body. None of these limitations exist in the beginning, but are gradually built up in the course of development and education. The little child is free from them. He knows no unbridgable chasm between man and animal; the arrogance with which man distinguishes himself from the animal is a later acquisition. In the beginning he is not disgusted at the sight of excrement, but slowly learns to be so disgusted under the pressure of education; he lays no special stress on the difference between the sexes, rather accredits to both the same genital formation; he directs his earliest sexual desires and his curiosity toward those persons closest to him, and who are dear to him for various reasons—his parents, brothers and sisters, nurses; and finally, you may observe in him that which later breaks through again, raised now to a love attraction, viz., that he does not expect pleasure from his sexual organs alone, but that many other parts of the body portray the same sensitiveness, are the media of analogous sensations, and are able to play the role of the genitals.”