Most cultures didnt know about the Hymen back then. They believed that if the female was "pure" or a virgin, she was to bleed during her consemation [sp].
The bleeding factor is indeed quite random, then again back in those days they married 12 year olds so the bleeding whilst having intercourse is probably caused by stretching more then anything.
As biology and the such came into light, they found the Hymen, and thats how parents would find out if their daughter was a virgin. They would ask a doctor to examine her to see if her Hymen had ruptured.
What you said about the different ways of a Hymen being torn and physical features is true, but your lecturer must be a fucktard to believe that some females are not born with a Hymen. Thats like saying a some males are not born with a foreskin.