The evils of lust is a theme in the work that is prevalent throughout and explored in many ways. Initially, the knight, Januarie, explains that—because of his rampant lust and desire—he wants to take a wife to allow him the chance at free, moral sex. This is frowned upon by the morally upstanding character Justinus, but Januarie goes through with the marriage anyway. Additionally, the increasingly perverse acts Januarie undertakes with May are implied to lead to his blindness. May and Damyan, too, fall victim to their lust, and their affair is revealed—though without real consequences.

Initially, Januarie marries not for love but to satiate his lust. Eventually, however, it seems that he grows to love May, as his speech becomes more ornate and flowery, showing metaphors of love as opposed to those of carnal knowledge. Damyan initially professes deep love for May, but they engage in a purely carnal affair. Ultimately, the pair of May and Januarie end up together because they, apparently, are the true lovers. Chaucer takes care to show the vacillations between lust and love and the ways in which relationships can slide between these two spaces.

The characters frequently act immorally, and gods like Pluto and Justinus reprimand their actions. There is no clear moral police, much less moral improvement. However, the story's undercurrent is that, through the experiences in the story, the characters might became more moral people and began to live in a more upright manner. The journey helps them to become better people and partners.