It isn't just Christianity that is rumored by its enemies to have more poorly lit incest than a Supernatural fan-fic convention. A minority Muslim sect called the Alevi faces widespread discrimination in Turkey to this day, because somehow this completely different religion has picked up the exact same rumor about pitch-black religious orgies. Clearly, something about unfamiliar worship services causes the human mind to make the jump to "They're taking their clothes off and sticking their dicks in each other," because variations of this rumor can be found far outside the Muslim world. In Europe from the 15th century onward, witches were the ones rumored to be having sex at secret nighttime religious gatherings. During the 1980s, Americans of all kinds became convinced that Satanist groups were stripping naked and getting it on in their local woods at night, which sounds like a great way to get your Dark Lord to bless you with Lyme disease.
Those robes are hiding a wicked poison ivy rash.