After unclean sex there are red spots at the base of the thighs a little pain, clinically common causes are ringworm, eczema, trauma and so on. 1. ringworm: the disease refers to the groin, perineum, around the anus and buttocks of the skin ringworm infection, which is mainly associated with direct contact with the sick people or indirect contact with contaminated personal items and fungal infections, the clinical manifestations for the affected areas of erythema, pimples, scales with itching, accompanied by the infection may appear pain. 2. Eczema: this condition refers to a chronic inflammatory itchy skin disease, which is related to contact with allergic substances, abnormal immune function and other factors, clinical manifestations of erythema, papules, oozing with itching, accompanied by infection may appear pain. 3. Trauma: improper force during unclean sex, not paying attention to protect the skin of the femur and other local damage, erythema, pain. It is recommended that people with the disease should go to the dermatology department of the hospital to complete the relevant examinations to clarify the diagnosis for further treatment.