Pegasi, Earth and Unicorn mares all suffer from what's known as a "Heat", when their libido reaches an apex that's borderline annoying. Some find temporary relief through friends, their special somepony or strangers. Other ponies use toys, their hoof or magic to satisfy their urges. Alicorns, however, are dignified royalty so they neither have time nor often desire to incite rumors among the populace by having such relations. Princess Celestia was such an Alicorn and, sadly, had no special somepony to sate her desires. She did, however, have a sister whom she trusted more than anypony else. A sister who was all too eager to aid in her sister's relief.