Hakushaku Reijou wa Kenen no Naka no Elite Kishi to Kyousei-teki ni Tsugai ni Saserareru

The duchy of Alita was invaded by the monarchy, the Empire of Sckinia, and is now under its rule. The children of the nobility of the former duchy were incorporated into the imperial educational system, where they are given the duty of becoming mates with a partner chosen by the empire in spring when they turn 18, and have children. Tiana, a daughter of a count, is chosen to become mates with an elite knight, Adel, with whom she's always butted heads with since they were children. Arguing and bickering with each other, their completely unromantic first night begins. Tiana thought this would be their first and last night together, but Adel calls on her night after night, saying it's their duty... A romantic fantasy story of mates who can't seem to be honest with each other! (Source: MangaPlaza)