The unchastity of a Vestal virgin was a religious offence punishable by death. According to Plutarch, a Vestal virgin was first executed for incestus during the reign of Numa Pompilius, the mythical second king of Rome (Life of Numa Pompilius 10.4). Yet Domitian’s execution of three Vestals for incestus in 83 CE indicates that this practice was observed well into the time of the Roman Empire.

Rome’s longstanding fixation on the sexual purity of the Vestal virgins is rooted in their symbolic function as the total embodiment of the city and its people. Not only was their chastity held to be representative of the state’s moral character, but their bodies symbolized the inviolability of Rome’s walls and the strength of her society. It follows that in times of great violence, the Vestals were occasionally suspected of incestus and sentenced to death. Such was the fate of Oppia in 483 BCE (Livy 2.42.9-11), when Rome was at war with the Veii and the Volsci, and of Opimia and Floronia in 216 BCE (Livy 22.57.3), amid the throes of the Second Punic War.

For a more extensive discussion, refer to Holt N. Parker’s article on the sexual status of Rome’s Vestal virgins.