The Lupercal Festival's Pagan Origins: The Lupercal festival was a pagan ritual associated with the god Lupercus, a Roman fertility deity. Flavius and Marullus, as stern and conservative Romans, likely disapprove of the festival because it goes against the traditional Roman religious beliefs and values. They may see it as a sign of moral decline and the erosion of traditional Roman customs and practices.
The Celebrators' Behavior: The participants in the Lupercal festival engage in rowdy and chaotic behavior, including running through the streets, whipping people with goatskin thongs, and engaging in boisterous revelry. This kind of behavior is likely seen as disorderly and unbecoming to Flavius and Marullus, who are known for their more serious and disciplined demeanor.
Contrast to Caesar's Ambitions: The celebration of the Lupercal festival also coincides with the ambitious political moves of Julius Caesar, who aspires to become the sole ruler of Rome. Flavius and Marullus may sense the danger posed by Caesar's growing power and see the festival as a distraction from the more serious concerns about Caesar's political motivations and the impending threat to the Roman Republic. They may believe that Rome's leaders are too preoccupied with frivolous celebrations and not paying enough attention to the more pressing issues of state.
In summary, Flavius and Marullus' annoyance with the Lupercal festival stems from their disapproval of the pagan origins of the festival, their disapproval of the chaotic behavior of the celebrants, and their concern about Caesar's ambitions and the distractions caused by the festival from the more serious matters of state.