Did it occur to you that a detail might have both symbolic and practical aspects?

Your professor was not trying to tell you that Bram Stoker chose to have Dracula climb down head first for this one reason and no other. Dracula could have left the castle by any number of creepy means. He could have transformed into a bat, or a mist. He could have jumped out the window and miraculously survived the impact, or floated unnaturally. Instead, Stoker chose a particular image - the count crawling like a lizard. He then described it with a word that was loaded with a suggestive double meaning: not just unnatural, but sexually "unnatural."

There is plenty of textual detail to suggest that sexual threat is a theme in Dracula. It is not at all a stretch to consider that a choice of a significant word might have been made to reinforce that theme.