Dorian's parents come to Skyhold to take him away based on a deal Dorian and his father made a long time ago that if Dorian was not in a relationship by a certain age, Halward could marry him to Livia Herathinos. No questions asked. Cullen only sees one way to ensure his chess partner stays in Skyhold. They only need to fake being in a relationship long enough to get Dorian's parents out of Skyhold anyway. How hard could it be to complete the Tevinter courtship rituals?