Kundera presents sexuality in terms of lightness and weight. The lighter characters of the novel are strongly erotic and view their sexuality as generative and creative. Sabina paints as imaginatively as she makes love; Tomas heals individual patients wit h the same precision that he uses to seduce individual women. Neither feels any degree of guilt for their sexual promiscuity. The heavy characters of Tereza and Franz, however, are marked by sexual guilt. Tereza hates bodies, especially her own, and Tomas 's extramarital affairs destroy her. Her one affair with the tall engineer is a disaster that almost drives her mad. Franz too is tormented, specifically by the thought that he must betray his wife for Sabina. He cannot bear to move from one woman's bed to the other, and ultimately leaves his wife to avoid deceiving her. He also refuses to use his physical strength in bed, stating that "love means renouncing strength."
Kundera also presents sexuality in gendered terms. Feminist critiques of Kundera have drawn attention to this fact, and level the accusation that in all of his novels, especially in The Unbearable Lightness of Being, sex is equated with rape or violence. Whether one considers the accusation valid or irrelevant in the context of the novel, the sexual comfort and joy the characters feel seems directly related to their acceptance of certain gendered behaviors. Unlike Franz, Tomas is sexually successful because he is not afraid to use his strength (a stereotypically male trait), but instead issues the infamous command, "Take off your clothes!" to women who almost always obey. Franz, who does not use his strength, is portrayed as weak, and a sometimes unsuccessful lover. Sabina, unlike Tereza, enjoys relinquishing power (a stereotypically female way of behaving). She thinks of the bowler hat as provocation, a "submitting of her own will to public rape." She enjoys the role of the stereotypically weaker woman, so she enjoys sex more than does Tereza. Tereza, contemplating her submission to the tall engineer, feels she must leave Prague or die of shame.
The body itself presents a separate paradox, as it cannot be clearly identified as heavy or light. The philosopher Parmenides would classify the body as heavy and the soul as light; however in a contemporary setting, those that choose the soul over the body seem the heavier. Of the four main characters, Franz and Tereza are the only ones who would even use a word like "soul." They search for a higher, nobler meaning of individuality than what they see in the body. Franz, after all, is a scholar and professor, and Tereza a political photographer. Both are interested in lofty ideas and convictions. Sabina and Tomas, on the other hand, are firmly rooted in and attached to the body. Sabina is a visceral painter, Tomas a doctor; both have many lovers and are fascinated by the human body.
In terms of sexuality, Sabina and Tomas are comfortable with their bodies and interested in those of others. Franz, on the other hand, considers his strong muscular body useless in a civilized world, and refuses to use his strength in a sexual context. Tereza goes further. She is revolted by the naked body, considering nakedness and sexuality potential horrors.
The body also figures as a symbol of death, as a corpse. Ultimately none of the characters are prepared for or have an understanding of death. Across all divisions of lightness or weight in philosophy, all the characters have reason to fear the dead body. The abandoned wife appropriates Franz's corpse, and the abandoned son appropriates Tomas's body. Tereza, for all her revulsion of the body, cannot escape becoming a corpse. Sabina, the only character left living, understands that her death will mean an end to all wanderings, and shudders at the thought of a heavy stone placed over her body.
The bowler hat, which Sabina describes as a leitmotif in her life, acts in the novel as a symbol for her eroticism and rebellion. The hat, which originally belonged to her grandfather, has been made (originally out of mischief) into an erotic plaything by Sabina and Tomas. When Franz removes the bowler from her head before making love, it symbolizes the stifling effect he has on what Sabina views as her most important characteristics, her unabashed eroticism and betrayal of roots.