In chapter five, Constance's affair with Michaelis is intensified when he asks her to marry him; she maintains that she's already married and doesn't really give him an answer, and at the end of the chapter the affair ends rather dramatically.
One of the remarks Lawrence has made frequently is that men finish too quickly, without a thought for their female partners. Luckily for us women of the real world, this is a broad generalization, often incorrect; but for Constance, it is a truth she lives with. She has somehow found a way to use men/Michaelis after they've come to achieve her own orgasm by holding them inside her and rubbing her clit against their base. She does this every time she has sex, because she deserves her satisfaction too.
At the end of chapter five, after they've had sex, Constance as usual finishes herself after Michaelis has already come, and he responds with this outburst: "You couldn't go off the same time as a man, could you? You'd have to bring yourself off! You'd have to run the show!" (page 56)
To Michaelis, Constance is the one with the power in their sexual relationship. Though he is the man, the phallus, the conquerer who sticks his flag in the sand and claims the land, he is essentially left behind while Constance pleasures herself in a place he can't reach, a summit he can't climb. Of course he feels emasculated and resentful. Constance, meanwhile, is completely baffled: "Because after all, like so many modern men, he was finished almost before he had begun. And that forced the woman to be active" (page 57).
Michaelis places all the blame for his bitterness on the women he's been with: "I never had a woman yet who went off just at the same moment as I did" (page 57). If she doesn't reach orgasm, it's like she's "dead in there," and if she comes after he does it's a struggle for him to hang on. Constance doesn't blame men for finishing too soon, because she's still able to go on and come; she has resigned herself to things in this order. Michaelis, however, is bitter about it; yet, he doesn't take any responsibility for being the one who finishes first. He thinks about a woman's sexual experience in terms of a man's, and that means a woman's orgasm should come on a man's schedule.
Michaelis' reaction to what Constance had thought of as the natural way of things has a disastrous effect on Constance: "It killed something in her. [...] There was nothing now but this empty treadmill of what Clifford called the integrated life, the long living together of two people, who are in the habit of being in the same house with one another. Nothingness! To accept the great nothingness of life seemed to be the one end of living." (pages 57-58)
Connie's sexual life is tied to the rest of her life, rather than being separated; each influences the other. Now, with the dismantling of her sexual knowledge--that is, the revelation that men expect women to come just as quickly--the rest of her life is become pointless and routine: an empty treadmill.
More on this as we read the next few chapters!