The left testicle swelling and pain after sexual fantasy may be a physiological phenomenon, or it may be caused by certain diseases (prostatitis, orchitis). 1. Physiological phenomenon: sexual fantasy will cause sexual excitement, resulting in genital congestion and edema, the left testicle transient swelling and pain, may be due to persistent congestion caused by the pain, the symptoms are not persistent, there are no other abnormal symptoms, do not need to worry and treatment. 2. Diseases caused by (1) prostatitis: belongs to the genitourinary system is one of the most common diseases, due to the inflammation of the prostate gland may cause the prostate gland congestion and swelling, sexual excitement or sexual life stimulation, may cause abdominal swelling, testicular pain perineum swelling and pain and other symptoms. (2) orchitis: bacterial viruses and other pathogenic infections may cause testicular inflammation, can also cause the symptoms of left testicular swelling and pain after sexual fantasy, most of them are also accompanied by testicular appearance of congestion, swelling, swelling and other manifestations. If the left side of the testicle pain, there may be other reasons, should go to the hospital in time to check, under the guidance of a professional doctor to follow the doctor’s instructions.