How would you address the head-trainer of your footbalteam; with "coach" or "trainer" or with his first name?
How about your physician, would you say doctor or use the first name?
Or the teacher at the university; "professor" or his first name?
Or your priest; "father" or his first name?
Your instructor in fencing; "maitre" or his first name?
How about your father? "Dad" or first name?