Foucault has suggested that the two major forms of bio-power are the discipline of the body and the regulation of population. Sex has become such a preoccupation in the modern world because it deals with both these forms of bio- power.
The four main lines of interest taken by sexuality all combine these two forms of bio-power. The interests in both child sexuality and female hysteria are directed toward a kind of discipline of the body: they try to control the behavior of children and women. Further, this discipline is enforced in the name of regulating the population. Children need to learn socially acceptable behavior, and women's health is closely tied to reproduction. The interest taken in birth control and sexual perversion aim at regulating the growth and behavior of the population (regulation of population), and do so by demanding certain forms of self-discipline (discipline of the body).
Foucault characterizes the transition between the right of death and power over life as a transition from a "symbolics of blood" to an "analytics of sex." Previously, blood was taken as a symbol of power. Blood lines and purity of blood were all important, the right of death was exercised by spilling blood, and so on. Now, power is exercised through sex. This interest in sexuality has rendered possible unprecedented knowledge, power, and control over a population. This transition was far from smooth, and Foucault identifies a symbolics of blood lingering in the racism of the Nazis and their demands for racial "purity." In psychoanalysis, sexuality is also read as being born out of earlier laws based on blood ties.
In all this talk about sexuality, has Foucault forgotten about sex itself? In portraying sexuality as a social construct, is he ignoring the simple, organic and instinctive fact of sex? Foucault replies that no; on the contrary, "sex" is more of a social construct than sexuality. When we talk about "sex," we are not talking about anything quite so objective as body parts, bodily functions, or physical sensations. We are speaking more generally about the meaning these things hold for us in particular contexts. Sex serves as a general causal principle that makes the deployment of sexuality possible. It is an ideal point invented by (and not existing prior to) the deployment of sexuality.
We have imbued sex and sexuality with so much importance that we now see it as the key to explaining who and what we are, much in the same way that past generations might have tried looked to astrology or metaphysics. We have become so caught up in the deployment of sexuality that we see the very possibility of our "liberation" as hinging on a healthy sexuality. The irony is that this belief that sex holds the key to our liberation is a manifestation of the power that is being exercised on us. If we want to resist this power, we should not focus on sexuality but on the body and the physical pleasures that sexuality tries to appropriate.
What is "sex"? Sex is not sexual intercourse; it is not the genital organs; it is not the pleasure derived from intercourse; it is not the gender people have; it is not their reproductive capacity. Certainly, all these things have something to do with sex: people can "have sex," and "male" and "female" are two distinct "sexes." But what is sex, the thing, itself?