Did Julius Caesar really get raped by the son of Crassus?

Dramatic Effect

To begin with the character of Tiberius Licinius Crassus is entirely fictional.

Marcus Licinius Crassus had two sons, Marcus Licinius Crassus (Jr in effect), and Publius Licinius Crassus.

So the issue is, was Caesar ever raped at all?

It seems unlikely as there is no contemporary record of such an event and if it were publicly known it would have been recorded and spread by Caesar's opponents.

Caesar was often subject to rumours of homosexuality but it has a different connotation now than it did in Roman times. It seems likely that this is the basis for the rape scenario portrayed in Spartacus here.

Wikipedia

Roman society viewed the passive role during sexual activity, regardless of gender, to be a sign of submission or inferiority. Indeed, Suetonius says that in Caesar's Gallic triumph, his soldiers sang that, "Caesar may have conquered the Gauls, but Nicomedes conquered Caesar." According to Cicero, Bibulus, Gaius Memmius, and others (mainly Caesar's enemies), he had an affair with Nicomedes IV of Bithynia early in his career. The tales were repeated, referring to Caesar as the Queen of Bithynia, by some Roman politicians as a way to humiliate him. Caesar himself denied the accusations repeatedly throughout his lifetime, and according to Cassius Dio, even under oath on one occasion

This rumour and it's sources and accuracy are remarkably well covered in the linked journal article linked below.

Certainly, later in life the Roman was regularly accused of having shared the king’s bed, and in a remarkable chapter of his biography (Jul. 49) Suetonius documents a number of instances.

^{Caesar & Nicomedes - J.Osgood (Source)}