Brun was aroused by the power he had over Ayla, by her fear and the reaction she had, trying to get away. When she becomes accustomed to it and has a "lets just get this over with" attitude, it takes away the power and his arousal and he gets embarrassed and can not get it up.
Because Broud was only turned on by Ayla's pain, fear and the knowledge that she hated it. Typical rapist behavior. The thought of hurting her aroused him but when he saw she was impatient and not afraid anymore he couldn't keep it up.