What is the most common cause of female sex dysfunction?

More than one. Causes of female sexual dysfunction include physical, hormonal & psychological/social. Arthritis, urine/bowel conditions, multiple sclerosis can all affect sexual function. So can more common fatigue, chronic pain including headaches, and surgery. Medication side effects play a big role. Both estrogen & testosterone are important in sexual function. Anxiety, depression, stress/worry & distractions.