Haikei Mishiranu Dannasama, Rikon shite Itadakimasu

Byletta, daughter of a viscount who has talents for swords and business, has been married to Arnald, son of a count, for eight years...however, he was away at war, and they hadn't met even once. When the war ended, she decides to ask for divorce even before she sees his face. She writes Arnald a letter saying "Dear Sir, I would like to ask for divorce." Arnald, handsome but had never loved someone, and Byletta, who was rumored to be a villainess, meet for the first time in their bedroom. He asks for a wager, and guarantees her that he will divorce if Byletta wins. If he wins, they will stay married forever. "If I can get you with child in a month, I win." Arnald says. That was something Byletta could never imagine.

(Source: Kadokawa)