What is the theme in The History of Sexuality: An Introduction by Michel Foucault?

The text is primarily concerned with the relationship between sexuality and power and with developing the history of sexuality. By "sexuality," Foucault does not mean sex or the physical experience of coitus; he often notes that sexuality is quite distinct from sex. Instead he defines sexuality as a purely historical construct that is difficult to easily understand - a network of the stimulation of bodies, the intensification of pleasures, and the incitement to discourse which forms a special knowledge. Thus, sexuality varies by time and place. The specific textual concept of sexuality is integral to an understanding of the thesis but, strangely, Foucault does not elucidate the meaning of sexuality until Part 4, Chapter 3.