In its current usage, sexuality refers to the cultural interpretation of the human body's erogenous zones and sexual capacities. That the same two sexes occur in every society is a matter of biology...that there is always sexuality, however, is a cultural matter. Sexuality is that complex of reactions, interpretations, definitions, prohibitions, and norms that is created and maintained by a given culture in response to the fact of the two biological sexes

"Originally the gentials were the pride and hope of living beings; they were worshiped as gods and transmitted the divine nature of their functions to all newly learned human activites. As a result of the sublimation of their basic nature...In the course of cultural development so much of the divine and sacred was ultimately extracted from sexuality that the exhausted remnant fell into contempt." -- Sigmund Freud, in "Leonardo da Vinci and a Memory of his Childhood," (Standard Edition, vol 11, p. 97) 

It was a main thrust of Freud's work to rescue sexuality, that "exhausted remnant" of cultural development, from intellectual and moral oblivion and contempt.

Freud's major work on sexuality is the Three essays on the Theory of Sexuality. In it, he extends the history of sexuality to a time before which it had been thought to be found and enlarges the scope of the concept beyond its ordinary definition. As he puts it, "In psycho-analysis the concept of what is sexual...goes lower and also higher than its popular sense. This extension is justified genetically." (XI,222.) (quoted in Wollheim, p.132)

Central to Freud's developmental account in terms of sexual aim is the concept of an erotogenic zone of the body. An erotogenic zone comes to be established when the pleasure associated with a somatic function (of mouth or anus, for example) becomes the basis for another instinct, on to which an object is "soldered" as a later accretion. (the analytic term for this process is Ahnlehnung or anaclitic) According to Jean Laplanche, "Sexuality in its entirety is the slight deviation, the clinamen from the function. It is the clinamen insofar as the latter results in an auto-erotic internalization." (Life and Death in Psychoanalysis, p.22) 

Sexuality in its earliest phases does not, Freud contended, possess an object. It consists in sexual aims which the infant endeavors to fulfill through its own body, and for this reason Freud applied to the whole period the term "autoeroticism." The subsequent attachment of an object is through the "anaclitic" route. (unlike affection, which starts with an object. see love
In his 1915 revision of the Three Essays, Freud identified different "organizations of the libido", the oral, anal, and genital, each referring to the erotogenic zone which enjoys primacy within it. Eight years later, in "The Infantile Genital Organization," he added the phallic, to indicate that, at this stage, although the genitals are paramount, only one type of genitals is recognized, the penis in boys, and, derivatively, the clitoris in girls. (Wollheim, p.123) Freud came to characterize the development of sexuality as a linear progression through the three pregenital organizations: the oral, the anal, the phallic, culminating in the genital. Freud's account continues with the original object choice, the mother, resulting in the Oedipus complex and the ensuing repression of sexuality around the age of six of seven, the period of latency, which ends with puberty. 

L.H.O.O.Q  Marcel Duchamp, 1919. The pronunciation is the same as “Elle a chaud au cul” meaning she has a hot ass

L.H.O.O.Q Marcel Duchamp, 1919. The pronunciation is the same as “Elle a chaud au cul” meaning she has a hot ass

In the essay on Leonardo, Freud deals with the relation between thinking and sexuality. Starting around the age of three, the child enters a period of "infantile sexual researches", when its curiosity is first aroused by the two great sexual questions: How are babies born? What differentiates the sexes? Like infantile sexuality, these thoughts succumb to a wave of repression, leading eventually to three possible outcomes: Thought, like sexuality, can remain inhibited (especially under the influence of religion). Thought can become a sexual activity, replacing sexual satisfaction but leading to (neurotic) compulsive brooding. Or thinking may to some extent liberate itself from its sexual past, which he calls sublimation. (see also eroticism) Freud characterizes Leonardo as an extreme example of the third type, in which the greater part of his libido escaped repression but was sublimated into a urge for research. Freud goes on to describe the smile of the Mona Lisa in terms of Leonardo's precociously erotic relation to his mother, which had also deprived him of a part of his masculinity. 

For Michel Foucault, the very category "sexuality" is a specifically modern construction, which did not appear before the nineteenth century. It is not just the modern interpretation of sex, but a new category -- central and centralized: universally organized as a tool for understanding, placing, and controlling individuals. As such, "sexuality" is one of the defining concepts of the modern subject -- a form in which individuals are able to recognise the "self." (It is in this sense that a collection of essays of "the construction of erotic experience in the ancient Greek world" can be entitled Before Sexuality.