The Male Sexual Response Cycle
Excitement: More so than with females, male sexual excitement can be generated by a much larger variety of stimuli. As it begins, the following processes take place:
Erection of the penis begins when hormones act to constrict the venous routes for blood out of the penis. This vasoconstriction results in pooling of the blood in the penis, thereby causing it to become engorged and erect.
The urethral opening widens
The scrotal skin becomes taut, and the testicles become elevated due to the contraction of the cremaster muscle in the scrotum. In some males, the nipples will become erect.
As with females, in some cases, males may experience a sex flush, whereby the skin in certain areas of the body become reddened due to the dilation of blood vessels.
The heart rate, breathing rate and blood pressure increase.
There is an increase in the tension of muscles throughout the body (myotonia).
Orgasm: As the male approaches orgasm, the following processes occur:
The male enters the "emission stage" of ejaculation. Ejaculation begins with the contraction of smooth muscle in the walls of the testicles, epididymides, vas deferens, ejaculatory duct, seminal vesicles, prostate gland, Cowper's gland, and the urethra, almost simultaneously. These contractions send semen into the urethral bulb. Simultaneously, a muscular sphincter that guards the opening of the urethra into the urinary bladder constricts to prevent urine from entering the urethra during ejaculation. The male senses this process as a sensation of imminent ejaculation, and usually portends a point of no return. If ejaculation fails at this point, the male may still enter the resolution stage and feel unsatisfied.
The "expulsion stage" of ejaculation begins next, with rhythmic contractions of the penis and the bulbocavernosus muscle at the base of the penis.
These contractions force the majority of semen out of the urethral bulbs and out of the penis. Typically, the first three or four contractions are the most intense, and are followed by a series of smaller, less intense contractions. Each contractions lasts a little less than a second. Read more on Hormones and Other Sex Chemicals
Resolution: The refractory period may or may not move directly into the resolution period – this varies. If a guy wishes to pursue further sex play, then he might return to the plateau stage and continue until he is able to once again become orgasmic. In the vast majority of cases, however, the guy will enter the resolution phase immediately after orgasm.
During the resolution phase:
Hormones act to release the constrictions on the venous system in the penis to allow blood to flow out of the penis, allowing the erection to subside. About half the erection is lost almost immediately, with the remainder disappearing over the course of 30 minutes to an hour. Heart rate, breathing rate, and blood pressure all return to normal.
The scrotum relaxes and returns to its normal state. The testes descend to their normal position. All of the other organs and appurtenances that have swollen (i.e., prostate gland, Cowper's gland, etc.) return to their normal size. As the prostate and other structures relax, remaining seminal fluid pooled there is pushed out into the urethra, and will leak out of the end of penis. The seminal fluid still contains sperm. The body releases generous amounts of prolactin, which causes the loss of interest in sexual activity in the near term (through suppression of dopamine functionality).
Although the phases are the same for everyone, the manner in which they move through them, both in terms of speed and the degree of difficulty, is as individual as the color of their eyes. It is important to understand how the sexual response cycle works so that you'll know how and why your body reacts the way it does to certain stimuli. Being in tune with your body and its natural cycles allows you to better embrace your own sexuality.
Dimension of Human Sexuality, 5th Edition, 1998
Human Reproductive Biology, 3rd Edition, 2006
The Science of Orgasm, 2006