A man’s sex organ (penis) is separate from his internal reproductive anatomy (testes and glands). The penis has three main functions.
Firstly men use their penis to urinate (but only when flaccid). The urethra is a tube that runs down the middle of the penis. When a man has an erection, the muscles at the base of the bladder contract and close off the urethra so it is impossible for urine to be released when the penis is erect.
Secondly the penis is the male sex organ. When a man has an erection, his penis becomes highly sensitised and pleasurable to touch. The erect penis juts out from the body at an angle of approximately 45 degrees. When the erect penis is stimulated, in conjunction with a mental focus on erotic stimuli, a man can achieve orgasm and he ejaculates. A man experiences pelvic muscle contractions and highly pleasurable sensations in the brain.
Thirdly the penis is a reproductive organ. Male orgasm triggers the ejaculation of semen, which contain a man’s spermatozoa (sperm). The male reproductive organs include the penis, the testes and the various male glands (including Cowper’s gland and the prostate). Intercourse is an activity that facilitates male ejaculation of semen. As a result of thrusting into the vagina, a man ejaculates and his sperm (from the testes) pass from the penis (via the vagina) into the uterus to meet the egg (produced by the ovaries). When a sperm cell and an egg meet and fuse together, a new life is formed.
Many mammals, including most primates, have a penis bone (bacula) that facilitates penetration. Females have an equivalent bone in the clitoris. Humans appear to be unique in having blood flow to the genitals, which in the male is trapped and causes the erection that makes penetration possible.
The post-adolescent penis is designed for penetration. It has a shaft including a core of erectile tissue, containing a specialized arrangement of arteries within a matrix of connective tissue. Two cylinder-shaped chambers (corpora cavernosa) run the length of the penis (either side of the central urethra). When these spongey chambers fill with blood, the penis becomes rigid and increases in size (both width and length). This creates an internal strength and rigidity that is far greater than would be possible in a hollow tube filled to an equivalent pressure. The corpora cavernosa are encased in a sheath of tough fibrous connective tissue (Bucks fascia). Between this sheath and the overlying skin is a layer of elastic connective tissue that allows the skin of the penis to move freely along the shaft. The penis and the clitoris extend back into the pelvis between muscles in the lower abdomen.
When a man is aroused, his brain sends messages to the nerves in the penis causing the arteries to relax and blood to flow in. The resulting pressure causes the penis to expand, which compresses the veins that normally allow the blood to drain away. Once the blood is trapped, a muscle within the corpora cavernosa sustains the erection. An erection is reversed when the muscles contract and stop blood flowing into the corpora cavernosa. It is common and quite normal for the penis to be curved (sometimes sideways). There is no detrimental effect on either reproduction or sexual pleasure.
The prostate lies at the base of the penis, just below the bladder. Before a man can ejaculate, he must be mentally aroused, which involves getting an erection. Thereafter he needs penile stimulation to ejaculate. Orgasm is the trigger for ejaculation. During male ejaculation, sperm is transmitted from the ductus deferens into the male urethra via the ejaculatory ducts, which lie within the prostate gland. It is possible for some men to achieve orgasm solely through stimulation of the prostate gland (stimulated via the rectum).
Some male prostitutes can climax five, six or more times per day regularly over many years. Even with such high frequencies, although the quantity of ejaculate is lower than usual, a little semen is always produced. Around three quarters of men do not ejaculate with any force. Their semen is merely exuded from the glans. Other men may ejaculate semen over distances of a few inches or even a few feet (very rarely up to even a meter or more!).
Having an erection, gives a man to have a strong desire to release sexual tension by thrusting into a body orifice. Intercourse involves a man inserting his erect penis into a woman’s vagina and thrusting until he ejaculates. The penis and the vagina act as reproductive conduits during intercourse. Male orgasm is a vital aspect of male reproductive function. The pleasure provided by orgasm triggers ejaculation and starts the reproductive process. A man may be inordinately proud of his penis as the source of his own pleasure and his hopes for arousing (or at least penetrating) a lover. Achieving penetration and ejaculation are key to men’s sense of well-being.
The role penetrator in intercourse involves substantially more pleasure than that of the receiver. Only relatively unresponsive men or women (who are incapable of penetrating anyone) offer a partner the pleasure of penetration. This is how responsiveness has evolved. Responsiveness involves an instinct to penetrate another person’s orifice (female erotic fantasies focus on the idea of penetration). Only a person who is born male (with penis and testes) can reproduce by ejaculating into a vagina. So only men have a sex drive.
The relaxed penis can be any size, depending on ambient temperature. (Antony Mason 1998)