Why can't someone make a good fuck film from this source material?  

Director Joe D'Amato hasn't failed, but this is not the definitive version.  The actor cast as Caligula looks like documentary filmmaker Ken Burns in drag, and sounds like the mysterious Austin Powers.  The sex is pretty basic, except for an indoor all-girl volleyball game that turns into a mini-orgy when two tribunes arrive.

A more decadent imagination could have saved this one.