We can see here that Missie May slept with Otis because of money and wealth.
Missie May's actions are influenced by the introduction of a glistening silver dollar, or "gilded six-bits," into their relationship by Otis Slemmons, a symbol of wealth and temptation. The coin represents prosperity and entices Missie May with the allure of luxury and material comfort, something she has not experienced before.
Missie May, growing up in modest circumstances, is tempted by the possibilities the coin represents. The idea of owning expensive items becomes appealing to her.