One of the central moral lessons of Frankenstein is that you shouldn't dress up in your bridal gown with a massively long train and turn your back to the window when there's a monster about. Also, don't let your dummy fiancée lock you in the room.

In the film, the monster doesn't do anything to Elizabeth; he just chases her around till she faints, and then the rescuers get in the door and he runs off. But the symbolism here—an invader in the bridal chamber—is clearly meant to point to sex.

Remember that Henry keeps putting the marriage off (and putting it off), but the monster has no such uncertainties. He comes right in. The monster then can be seen as all the buried sexual instincts that Henry has pushed to one side—instincts which come back, despite his best efforts, and wreak havoc.

There's also the symbolic suggestion that Henry and the monster are the same person. Remember, Henry locks Elizabeth in the room; he ends up helping the monster. Man and monster collaborate to trap and assault her. Henry is supposed to be a pure souled romantic hero at this point in the narrative. But the return of the monster indicates that, perhaps, he's still kind of a stinker.