Sexual desire disorders or decreased libido can be caused by a decrease in normal estrogen (in women) or testosterone (in both men and women) production. Other causes may be aging, fatigue, pregnancy, medications (such as the SSRIs) or psychiatric conditions, such as depression and anxiety.
There may be medical causes to these disorders, such as decreased blood flow or lack of vaginal lubrication. Chronic disease can also contribute, as well as the nature of the relationship between the partners. As the success of sildenafil (Viagra) attests, most erectile disorders in men are primarily physical, not psychological conditions.
It is unclear exactly what causes vaginismus, but it is thought that past sexual trauma (such as rape or abuse) may play a role. Another female sexual pain disorder is called vulvodynia or vulvar vestibulitis. In this condition, women experience burning pain during sex which seems to be related to problems with the skin in the vulvar and vaginal areas. The cause is unknown.
An erection occurs when the nervous system activates a rapid increase in blood flow. The vascular muscle in the spongy area becomes engorged with blood and the outflow of blood is cut off. An erection can occur as a reflex as we see in spinal cord patients, or can be caused by psychogenic (originating in the mind) stimulation. Numerous sexual stimuli are processed by the brain and transmitted to the penis via the nervous system.
Erectile dysfunction, also called impotence, means that a man's penis doesn't get hard enough to have intercourse. The man cannot get or maintain an erection. This condition affects approximately 30 million men in the United States. Erectile dysfunction is not the same as premature ejaculation, the inability to ejaculate (retarded ejaculation) or infertility.)
Nerves must be working normally for a man to get and keep an erection. Nerve damage can result from diabetes, multiple sclerosis, prostate surgery or damage to the spinal cord.