An excerpt from 25,000 Years of Erotic Freedom, by Alan Moore:
Sexual openness and cultural progress walked hand-in-hand throughout the opening chapters of the human story in the West, and it wasn't until the advent of Christianity, or more specifically of the apostle Paul, that anybody realized we should all be thoroughly ashamed of both our bodies and those processes relating to them. Not until the Emperor Constantine had cut and pasted modern Christianity together from the loose scraps of Mithraism and the solar cult of Sol Invictus, adopting the resultant theological collage as the religion of the Roman Empire, did we get to witness the effect of its ideas and doctrines when enacted on a whole society.
If we take a traditional (and predominantly Christian) view of the collapse of Rome, then conventional wisdom tells us that Rome was destroyed by its decadence, sunk beneath the rising scumline of its orgies and of its own sexual permissiveness. The merest skim through Gibbon,* on the other hand, will demonstrate that Rome had been a heaving, decadent, and orgiastic fleshpot more or less since its inception. It hgad fornicated its way quite succesfully through several centuries without showing any serious signs of harm as a result. Once Emperor Constantine introduced compulsory Christianity to the Empire, though, it barely lasted another handred years.
Largely, this was because Rome relied on foreign troops - on cavalry from Egypt, for example - to defend the Empire against the Teutonic hordes surrounding it. Foreign soldiers were originally happy to enlist, since Rome at that point took a pagan and syncretic standpoint that allowed recruits to worship their own gods while they were off in northern Europe holding back the Huns. Once the Empire had been Christianized, however, that was not an option. Rome's new Christian leaders decided it was their way or the stairway, and si consequently, off in distant lands, recruitment figures plummeted. The next thing anybody knew, there were barbarians everywhere: the Huns, the Franks, the Visigoths, and worst of all the Goths, with their white contact lenses and Cradle of Filth collections. Rome, effectively, was over, bar the shouting.
* I'm not personally sure who Gibbon was at the moment. - Neil