Don't even need to go that far in.
- Enter Transylvania from any other travel location.
- Enter the Succubus Tower, and Rinny will excitedly bounce up to you.
- Tell her "No" 6 times, and a list of all 6 options will appear, along with some new ones.
- Select "Have her grow a cock" at almost the very bottom of the list, watch the scene, and ride her to climax.
This can even be done upon entering the tower for the first time, after answering if you were there just to see her or not.