In questioning the repressive hypothesis, Foucault is not primarily interested in contradicting it, and he certainly does not want to deny the fact that, for instance, sex has been a taboo subject in Western culture. His interest is primarily the "discursive fact" of sexuality: he wants to know how and why sexuality is made an object of discussion. Ultimately, his interest is not in sexuality itself, but in our drive for a certain kind of knowledge, a certain perspective, and the kind of power we find in that knowledge.
Foucault uses the word "discourse" frequently, and has a very specific meaning in mind. When we talk about a "discussion," we are talking only about what has been said. When we talk about a "discourse," we are talking also about who has done the speaking, how they have done it, in what context, in reaction to what, and so on. The term "discourse" takes in the wider context in which words are uttered.
Discourse is important to Foucault because to him, language and knowledge are closely linked to power. Speech and writing are not simply the communication of facts that occurs in a vacuum. As important as what is said is who decides what is said. Foucault develops a complex body of thought out of the old saying that "knowledge is power." Whoever determines what can be talked about also determines what can be known. Whoever determines what can be known effectively determines how we think and who we are. According to Foucault, then, language and knowledge always have a political edge.
According to the repressive hypothesis power has been exercised to repress discussion of sex. More important than sex, though, is the discourse on sexuality. The institution of marriage has claimed the discourse on sexuality as its exclusive property: it has complete power of what is and is not said about sexuality. Effectively, culture bans any discourse on sexuality that occurs outside the confines of marriage.
The repressive hypothesis explains why the institution of marriage claims exclusive rights to discourse on sexuality. This hypothesis links sexual repression to the rise of the bourgeoisie. Unlike the aristocracy that preceded it, the bourgeoisie became rich through work and industriousness. Such a class would value a stern work ethic, and would frown upon wasting energy on frivolous pursuits. Sex for pleasure, then, became an object of disapproval, as an unproductive waste of energy.
Discourse, power, and knowledge are all linked in this hypothesis. On the one hand, those who are in power, the bourgeoisie, control discourse. They decide how sex can be spoken about, and by whom, and so they control also the kind of knowledge we have regarding sex. On the other hand, this control over discourse is closely linked to their maintenance of power. The bourgeois would want to control and confine sex because it is a dangerous opponent to their work ethic. The desire to control discourse and knowledge about sex is essentially a desire to control power.