Almost any condition that can cause an ulcer on the skin elsewhere on the body can also cause an ulcer on the genital skin. A common example is the bacterial infection of a minor skin irritation to cause impetigo (school sores).
An injury to the area is an obvious cause, but sometimes injury may be forgotten if it happens in the heat of a sexual encounter, especially if artificial aids are used to increase sexual stimulation.
Skin cancers are a possibility that should not be overlooked.
The most common reason for an ulcer to develop on the penis and scrotum of the male, and on the vulva and in the vagina of a woman is a venereal (sexually transmitted) disease. These include genital herpes (causes a blister which bursts to form a painful, tender, shallow ulcer), syphilis (causes a painless ulcer at the original site of infection), and rarer venereal diseases such as chancroid, donovanosis and lymphogranuloma venereum.
Behçet syndrome is a rare condition that causes eye inflammation, arthritis, genital ulceration and sometimes convulsions.