Watashi, Aishi no Outaishi-sama no Sokushitsu Yametai n desu!

Rose Marie has spent most of her life in the royal harem of Glenfil, as the first concubine of Crown Prince Ulysses, until one day she realizes that she can't bear to stay there any longer. But no sooner has she made that decision than she gets the order to complete the "Rite of Consummation"...! Though she's technically the most senior member of the harem after her ten years of residence, due to her age she hasn't yet been recognized as an official concubine. Once she completes this rite, though, she really will be stuck in the harem forever. Rose Marie is now scrambling around trying to find a way to get out of this duty, but Prince Ulysses won't let that happen...?!
(Source: MangaPlaza)