Comment on Lawrence's frank treatment of sexual desire in his novel The Rainbow.

Lawrence took the topic of sex very seriously indeed. (A little too seriously, perhaps.) He saw it as the expression of our most fundamental being, a powerful, subconscious drive that transcends the merely animalistic desire to propagate the species. Indeed, for Lawrence, sex for the purposes of procreation is a sideshow, representing as it does a crudely reductive understanding of one of life's great mysteries.
In The Rainbow, Lawrence portrays sex as the gateway to the eternal. According to Lawrence, who draws extensively on the philosophy of Schopenhauer, it is only through the act of sex that we can briefly take leave of our material selves and enter into a higher world. In engaging in sexual relations, we participate in the eternal, which is where we originally came from and where we will return once we die.
For Lawrence, women have greater power than men to apprehend the mysteries of the eternal and establish an earthly bridge to them. Sex is one way, and indeed the most important way, of establishing such a connection between this world and the next. That would explain why it's the female characters in The Rainbow who are the most overtly sexual. An especially telling line in the book expresses this point neatly:

He was nothing. But with her he would be real.

This refers to Tom's meeting Lydia. The implication is that Tom, like all men in the modern world, has forgotten how to think with his blood, as it were, to partake of those mysterious forces to which women are always in such clear proximity. The modern world, with its reductively scientific view of everything, including sex, has drained all the mystery from men's lives. Yet the mystery of life in all its fulness still exists—only now it's women, like Lydia and Ursula, who act as keepers of the flame. The primary means by which they keep the flame of mystery alive is through the power of sex.