Hello,
It can be due to rash due to friction rub during intercourse or fungal infections. Wash the areas several times with fresh water. Do not use any cosmetic products at the sites. You can apply some calamine lotion at the rash as it will help in soothing the skin.Also wear cotton undergarments.Application of mild corticosteroid cream and oral antiinflammatories like ibuprofen/acetaminophen are also helpful.If the symptoms are severe then it will be best to consult a dermatologist
Hope it helps.Take care and regards.
Could be a fungal infection (thrush) - taking oral fluconazole (available from pharmacies without prescription in most countries) should clear it up.