18+ Gensui Koutei no Okiniiri: Botsuraku Reijou wa Kakoware Dekiai ni Honrou saretemasu

One day, Roseline, the daughter of an indebted lord, helps an injured knight named Clemens. Although he does not reveal his identity and seems to be involved in the war for the throne, Roseline quickly falls in love. After a turn of events, he unbelievably becomes the new Emperor! As soon as he conquered the capital, he came and marries Roseline. They spend many nights together, teaching each other pleasures with their bodies. However, no matter how much love they have for each other, being the daughter of a dishonored noble could become an obstacle in becoming an Empress...

(Source: Harper Collins Japan, translated)