The Best Bride in the West

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)