General promiscuity or birth control I can't say anything about but according to Alastair Blanshard the idea of the Roman Orgy was a much later invention. Christian writers imagined that Pagan religious rites were large scale sexy parties when they clearly weren't. This was just a religious misunderstanding. This theme was picked up by writers like the Marquis de Sade much later on who let their imagination run riot with the sorts of depravities that the Romans must have been engaging with (but actually never did). It's only a short jump over to Tinto Brass and we have the modern template for the Roman Orgy, even if it has no historic precedent.

Sex: Vice and Love from Antiquity to Modernity (Blackwell Publishing, 2010)

Classics on Screen: Ancient Greece and Rome in Film (Bloomsbury Academic, 2011)