In Book 5 of Ovid's Metamorphoses, the poet tells the story of how Pluto, god of the underworld, abducted Proserpina, the daughter of the goddess Ceres. Ovid was not the first epic poet to take up this topic, as a Greek version of this story can be found in the Homeric Hymn to Demeter, which was written about 500 years before Ovid.
The humor that one finds in Ovid's account of the rape is often a subtle, refined sort of humor, not the sort of "ha-ha" humor one might find in a comedy sketch on television or at a comedy club. Sexual assault is no laughing matter, but Ovid, unlike his Greek predecessor, moves the focus away from the pain and suffering of the goddess and her daughter, and draws the audience's attention to other matters.
One example of the subtle humor in Ovid's version of the story, Venus has her son Cupid shoot Pluto with an arrow to cause him to fall in love. Venus does this because she is upset that her "empire of love" does not yet extend to the underworld. So, in essence, Pluto's rape of Proserpine is a result of Venus being power hungry.
Another example of Ovid's playfulness is his inclusion within the narrative of stories like that of the boy who laughs at Ceres, who then turns the boy into a newt. Ovid also, in the course of this narrative, tells of the transformations of Cyane, Ascalaphus, the Sirens, and Arethusa. Indeed, Ovid's interest in these tangential stories nudges the audience's attention away from the sexual assault and toward less serious matters such as people being transformed into birds or bodies of water.
Ovid even concludes this narrative on a positive note:
Now the goddess’s [Proserpine's] looks are glad that even Dis [Pluto] could see were sad, a moment ago. Just as the sun, hidden, before, by clouds of rain, wins through and leaves the clouds. (A.S. Kline translation)
This is a good question. Let me give you a brief summary and then look at the potential humor.
Within this story Cupid makes Dis (god of the underworld) fall in love with Proserpina. As Proserpina is gathering flowers from a grove, Dis appears and rapes her. Then he takes her to his underworld kingdom. However, there is a witness, Cyane, a nymph. When Ceres, the mother of Proserpina, searches for her, Cyane tells the whole story. At this point, the land of Sicily feels Ceres’ anger, as she is the goddess of the harvest.
After grieving, Ceres decides to go to Jupiter. Jupiter tries to defend the actions of Dis, but it is to no avail. Jupiter says that as long as Proserpina has not eaten anything, she may come back. Unfortunately, Proserpina has eaten something. So, there is a compromise. Proserpina will spend her time equally between Dis and Ceres,
There seems to be three humorous elements here. First, Jupiter tries to cover for Dis, his brother. He says that he raped her out of love. This is ridiculous. Then Jupiter tries to encourage Ceres by saying that at least Proserpina married well! Again this is outrageous. What is most humorous is that the leader of the gods is saying these words. Here are the words of Jupiter from Ovid's text.
"This daughter is a care, a sacred pledge to me as well as thee; but if it please us to acknowledge truth, this is a deed of love and injures not. And if, O goddess, thou wilt not oppose, such law-son cannot compass our disgrace: for though all else were wanting, naught can need Jove's brother, who in fortune yields to none save me. But if thy fixed desire compel dissent, let Proserpine return to Heaven;"