Seibu Ichi no Hanayome

Monica promised her childhood friend Leon that she would marry him if he became the best gunfighter in the West. Their marriage goes off without a hitch, but on their wedding night, when they were supposed to be at the height of their happiness, Leon tells her he wants a divorce. What is the cause of the complication and what will happen to them in this lighthearted romantic comedy? (Source: Shonen Jump Plus, translated)