The genitals (penis and scrotum in the male, vulva and vagina in the female) may itch for similar, or totally different reasons in males and females. Any skin condition that causes itching can also affect the genital skin. Common examples are eczema, reactive dermatitis and psoriasis.
If you have an itch of any cause, and scratch it, the scratching will further irritate the skin, leading to yet more itching and scratching. The condition becomes self-perpetuating, although the cause of the original itch has long gone. If it itches, do NOT scratch it!
Excessive sweating in an area that is usually well covered and constricted by clothing is a common cause of skin irritation and itching. The damaged skin may become infected by fungi and/or bacteria to cause a painful, oozing rash. Prevention is better than cure, and regular washing of the area when sweaty, loose clothing and cotton underwear (nylon may look sexy, but is not good for skin) can all help.
Over washing of the area on the other hand, particularly with strong soaps, can remove all natural protective oil, and lead to dry itchy skin. Minimal use of soap and thorough rinsing is a better approach.
Other causes common to both sexes include an allergy reaction (eg. to soaps, clothing, antiperspirants, toiletries, perfumes, contraceptive creams, lubricants etc.), infestation of the pubic hair (eg. with scabies, lice or crabs), genital herpes, genital warts (caused by the human papilloma virus) and poorly controlled diabetes (due to excessive sweating and superficial infections of the affected skin by fungi and bacteria).
Psychiatric conditions, including depression, may often include itching of the more private parts of the body as one of their symptoms. This may be because the mind becomes focussed inwards, magnifying minor irritations, and excluding the outside world.
Conditions that may cause genital itching in women include thrush (fungal infection of the vagina), vaginal infections by bacteria or parasites (eg. Trichomonas), excessive natural vaginal secretions (leucorrhoea often due to excess oestrogen), infection of the bladder (cystitis), urinary incontinence (urine can irritate the genital skin), cancer of the vulva may first be noted as a hard area of itchy skin, and a lack of oestrogen in older women after the menopause can cause the vagina to become dry and itchy. The burning vulva syndrome is a rare condition that causes exquisite tenderness and itching of the vulva, but its cause is unknown.
In men a genital itch may be due to fungal and bacterial infections under the foreskin of the penis, venereal diseases that cause a penile discharge (eg. gonorrhoea, chlamydia) and rarely cancer of the penis.