Some sexologists, e.g., Masters and Johnson, divide human sexual response into the stages of “excitement, plateau, orgasm, and resolution”. This description focuses almost exclusively on the physiology of sex.
Others, such as Helen Singer Kaplan, criticized this view for its lopsided focus on physiology, and emphasized the importance of psychological (emotional and mental) factors. This led to a division of human sexual response into “desire, arousal, and orgasm”.
Besides broadening the focus, this division also responds to the difficulty of distinguishing between Masters and Johnson's “excitement” and “plateau”, other than saying the latter is a more intense version of the former. Kaplan's "arousal" seems to include both excitation and plateau.
Rosemary Basson has a more persuasive “circular” – rather than “linear” – approach to woman’s sexual response, which is so compelling that I have highlighted it in a separate section here. She points out, among other things, that in women (especially in long-term relationships), desire may not precede arousal.
My own view would draw on all of these models. (Keep in mind that my focus is on a certain conception of married sexual intimacy.) It would describe the human sexual response model as: "desire-early arousal-high arousal-sexual union-orgasm-resolution."
Like Kaplan and Basson, I would stress the importance of non-physiological factors, especially "desire" -- which is at least as much psychological as physical.
I would also distinguish, however, different stages of arousal. (Perhaps this may overlap partly with Masters and Johnson's excitation-plateau distinction, but not much, I think -- because I don't view "excitation" as simply a physical process.)
I think the sexual response often begins with a fairly generalized form of desire, which may be physical (especially in men) or primarily emotional (especially in women). This is a general (not intense) desire for sexual union or a general (not intense) receptivity to it.
That desire leads, if circumstances are propitious, to the early stages of physical marital intimacy -- foreplay. Foreplay is the process of moving from desire to arousal. It is “getting aroused” – because arousal is a process, a movement (though sometimes a very quick movement), in which a certain sense of urgency -- an increasing desire to move forward in the act -- builds up. The very goal of foreplay is to arouse, to prepare the hearts and bodies of the couple for the ultimate physical sexual union – for the insertion of the penis into the vagina.
Foreplay, I think, is not just "fore" -- before -- sexual union (intercourse); it is before complete or high arousal. While arousal is a process, there does seem to be a definite moment when a woman “gets into it”, when she becomes fully absorbed in sexual arousal. She is able to converse at earlier stages, during foreplay, but at this point that ends. There is nothing “fore” about this stage – she is deeply into the sexual arousal at which "foreplay" aims. So I think that it is necessary to distinguish between "low or growing arousal" (during foreplay) and “high arousal” or “complete arousal”, which follows foreplay and precedes sexual union proper. [This distinction also leads me to consider it misleading that on the internet a search for images of “foreplay” typically result in many pictures that are really “post-foreplay”.]
This stage of high arousal can be quite sustained, especially in a woman. It is a source of overwhelming, absorbing pleasure for the wife and of overwhelming satisfaction for the husband, representing as it does his success at bringing the woman he loves to this high and continuing point of pleasure. It often includes a woman's first orgasm in this act of marital intimacy.
The stage of high arousal is followed by that of sexual intercourse in the strict sense, that is, sexual union -- the union of the bodies of the husband and wife, through the insertion of the penis into the vagina. This leads to the completion or fulfillment of the act in orgasm, especially the husband's orgasm and ejaculation. (In some cases, where earlier sexual activity has not brought the wife to orgasm, a husband may continue to stimulate the wife until she achieves orgasm.)
"Sexual intercourse" has a stricter sense -- the union of the bodies through the entry of the penis into the vagina -- and a broader sense, which includes the activity leading up to the entry of the penis (foreplay and sexual stimulation prior to entry). (Only relatively recently in history, has "sexual intercourse" come to be used by some to apply to oral and anal "intercourse" and to mutual masturbation.)
After the male (and sometimes female) orgasm, both husband and wife enter into the phase of resolution, in which they return to a pre-arousal stage, usually with a feeling of well-being and closeness.
This description of human sexual response will be elaborated on more fully in the section on marital lovemaking, but this section at least lays out the basic outline of human sexual response, as I see it.