gender input loop keeps repeating even if I enter "m" or "f"

If it's not m or f, repeat prompt until it's either or. I typed m or f, but the loop keeps repeating. Why?

gender = input('m/f? ')

while gender != 'm' or gender != 'f': 
    print("Type 'm' or 'f'") 
    gender = input('m/f? ') 
if gender == 'm': 
    gender = 'him' 
elif gender == 'f': 
    gender = 'her' 

print("It read {}".format(gender))