Technically, yes, Doo's actions would be considered rape, but this particular portion of the film took place in an era where this sort of behavior, though frowned upon today, happened quite often. Loretta was not only a young teenager but this was also her first time away from home and her family and this was probably very confusing and frightening. She screamed 'no!' numerous times, but in that day and age, nobody really would have intervened.