It is very hard to pinpoint the exact cause of rashes in the private part/genitals for that to be done you have to visit a doctor and have your medical history taken and perhaps also some laboratory tests.
Having said that below are some of the reasons as to why someone may have a genital rashes
Pubic lice are tiny insects. They lay eggs in the genital area and are most often spread from person to person through sexual contact. They are most commonly seen in teenagers. An infestation of pubic lice causes itchiness and sometimes sores.
Body lice are different from pubic lice and are larger. They live in clothing and on the skin and feed on blood. They cause an itchy rash on the skin.
Scabies is an itchy skin rash that is caused by very small mites. They burrow into the skin and cause intense itching, especially at night.
Allergies and autoimmune disorders are other possible causes of a genital rash:
Contact dermatitis is a common type of rash caused when skin comes into contact with an allergen or with an irritant like a harsh chemical substance. E.g. some women are allergic to some scented sanitary pads.
Psoriasis is a common skin condition. It is suspected that it’s an autoimmune disorder. It can produce a pinkish, scaly, itchy rash anywhere on the body.
Reactive arthritis, or Reiter’s syndrome, is arthritis that occurs in reaction to an infection by certain bacteria, such as Chlamydia, Salmonella, or Shigella.
Sexually transmitted infections are equally a likely cause of genital rashes