In her late teens, Princess Celestia goes to her father for advice on how to handle the conflict between her duties as a princess and her desires as a young mare. Fortunately or unfortunately for her, King Starfire is wise, understanding, and a bit of a Troll Dad. Celestia, as the Princess Heir, is destined to be Ruling Queen of Equestria, and the Ruling Queen's husband will be King Consort, and for this reason, absolutely must be a suitable pony to be a king. This, of course, completely rules out unstable, radical chaos mages with no family or even species who aren't ponies and really enjoy stirring up trouble. However, according to King Starfire, there are some very good reasons for a Princess Heir or a Ruling Queen to take a lover who cannot be king.
Or, in other words, "If you want Discord, take him, just be discreet about it."
Discord/Celestia. Adult.
Side story to The Last Draconequus.
The Most Embarrassing Conversation Ever