Kate was a whore. She enjoyed being a whore. She was a whore even when she didn't need to be.
Kate saved John's life in Firt Griffin when he had been bound in a hotel waiting to hang for killing a popular gambler. She had fired a stable and freed him. When they reached Dodge, John believed he owed her a life debt and was responsible for her. He settled 'permanently' in Dodge as a respectable dentist with a respectable 'wife.'
Two weeks later she was working in a brothel again and brawling in the street. He was ashamed.