Hello,
It can be due to friction rub during intercourse or contact dermatitis to the material of the condom or diaphragm or your partner’s soap or cosmetics. 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.
I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
You didn't say, but if you are using a condom, most are made of latex (sheep) and can promote allergies.
Process of elimination... it only happens with sex, cortisone took it away... it is possible you both have a latex allergy.
A dermatologist would have to say for sure.