The title of the episode, "Red Dirt", refers to what Jeremiah says about the Earth under the ranch being filled with blood.
When Madison tells Alicia that the "rhythm method" doesn't work, she is referring to a form of contraception that involves keeping track of the woman's ovulation cycle and abstaining from sex during ovulation. It is not an overly accurate method of birth control, it requires detailed and accurate record keeping. Plus there are many factors that can alter when exactly a girl/woman starts to ovulate, and stops, such as stress, changes in diet, malnutrition, which are things commonly experienced in an apocalyptic world such as the one Alicia finds herself in.
This is not the first time that Madison hesitates to kill a zombie version of someone she knows. She did the same thing back in Season 1 when she couldn't bring herself to kill her neighbor, Mrs Tran.