To Sir, Without Love: I'm Divorcing You, Vol. 1 (manga)

On the day Byletta is supposed to meet her husband-to-be...the man’s gone off to war! Eight years later, as she’s enjoying her spouseless married life, Byletta receives news that he is set to return. Determined to keep her man-free lifestyle, she sends a divorce letter. But the man (whom she still hasn’t met) responds with an outrageous wager: to sleep together for a month and she’s free to go if she doesn’t conceive!