To be clear, I'm well aware of the fact that Starz' Spartacus turned the sex and violence up to 11. However, the idea that the Romans were particularly decadent and cruel has been around for a long time. So, how much casual murder and rampant sex (including rape) was actually going on, if at all? If it was, during what periods?
While watching the show, one thing I noted was that a character implied that the elite could get away with a lot more in Capua than in "prudish" Rome. Was this true? Was Capua the Las Vegas of the Roman empire?
The idea of Roman "decadence" actually has a somewhat strange history. It's root can be found in an extensive body of moral literature produced by Roman authors which often took bodily depravity as a theme. Juvenal and Seneca were well known and fairly straightforward practitioners of this genre, but you can also find more interesting examples like Lucian's "The Dependent Scholar" (Περὶ τῶν ἐν Μισθῷ συνόντων ) which describes the life led by a scholar attached to prominent Roman households. However, these cannot be taken at face value, as they exist as critiques of society from within the society and were not meant as complete portraits of Roman life.
The aftermath of Roman literature, however, was through monasteries and the works that were transmitted tend to be those that fit a particular purpose. In this case, the purpose would be demonstrating the moral failings of pagan society, as well as the general moral programs of monastic society. The grotesque portrait of Rome painted by Juvenal was extremely popular for just this very reason. The specific critiques of aspects of Roman society was thus transformed into a general critique of non-Christian morality.
The final step then comes with the rise of pornography. This is something /u/vertexoflife has talked about, but pornography would often be set in some sort of exoticized location--there has been a lot of work on how "the orient" was eroticized through this, but also things like the eroticization of Roma or other travelers. And when looking for a setting, what place was available to pornographers that was both very familiar and deeply associated with a highly sexually charged body of literature? Ancient Rome! A huge amount of erotic art and literature takes place in ancient Rome (for example, the works of Paul Avril), and paintings set in ancient Rome would often have an erotic element, for example Alma-Tadema's paintings which often emphasize the sensual liberality of the Roman bath.
There are many different "Romes" in the collective historical conscious, often rather contradictory. Through the twists an turns of historical circumstance, one of these Romes is one of a highly sexual nature.
