After reading Three Contributions to the Theory of Sex, it's easy to understand why he shocked the Victorian world. We might take his views in stride today, but asserting that children had sex drives and elaborating forms of "perversion" was a stark departure from the climate of his time.
Sex Theory is divided into three sections. The first claims that sex "aberrations" can be explained by understanding the interactions between the infantile sex instinct and their restraining forces, shame, loathing, sympathy, morality, and authority. The aberrations include fetishism, sadism, masochism, voyeurism, and inversion (homosexuality). Psychoanalysis can understand the causes of these aberrations by uncovering unusual events in the psycho-sexual development of the child resulting in either "inhibited development" or "dissociations from the normal development."
Today, one might question the moral judgements (implicit or explicit) in Freud's account, particularly with regard to homosexuality, as today we recognize a wider diversity of sexual behavior as healthy and normal. Nonetheless, Freud's observation that infantile sexuality has some significant affect on adult personality stills seems plausible. In the second section of Sex Theory, Freud describes various stages and characteristics of infantile sexuality. It may have been less shocking to his time had he called this "carnal satisfaction," since much of what he described was considered to be quite distinct from adult sex. Nonetheless, describing it in these terms establishes the connection between infantile and adult sexuality and expands the boundaries of what might be understood sex. Both consequences are important to understanding his theories.
The third section, entitled "The Transformation of Puberty," explains the important phase in which the diverse streams of infantile sexuality are united into "one unit, one striving, with one aim" to become adult sexuality. At the same time, he recognizes that perversion and neurosis (perversion's "negative") are present to some degree in all adults.
After reading Three Contributions to Sex Theory, I can appreciate how Freud has been identified as one of history's greatest theorists. Whether his particular theories are valid or not does not detract from the rigor of his analyses and the creativity of his approach to understanding the human psyche. It seems quite appropriate to rank him with Aristotle, Galileo, and Darwin.