The question posed was:
From p. 43 of "The Way to Happiness: A Common Sense Guide to Better Living", a 330 page hardbound text used in the Narconon/scientology program:
"The problems of sexual misbehavior are not new.
The powerful religion of Buddhism in India vanished from there in the seventthcentury. According to its own historians the cause was sexual promiscuity in its monasteries."
Does anyone know what Hubbard is talking about here?
My answer does not answer precisely to your question, but more generally to some thoughts about the presentations done by scientology/hubbard of "religions and religious facts.
Scientology/Hubbard have spent some time trying to ridiculize religions o great men of the past, or to add their own versions. These are exposed is the first part of WIS (with flat pictures/photos to illustrate the short texts explaining scn viewpoints.
To begin with, no chance that the greek letter theta could have meant "spirit". Theoria (in greek letters) meant 'contemplate' which gave 'theory', and the fact that 'theta' was its initial letter does not prove anything. Theta came from a phoenician letter meaning "snake". Not a lot of correlations possible with a spirit, unless with Hubbard's one...
Regarding past lives, Hubbard or his scn researchers seem to have attribute them to egyptians, but that belief was certainly anterior -see Geetz and Bergounioux study of religions.
Hubbard/scnists did also try to explain that if Rome condemned the Christ it was mostly for political causes like "revolutionary thoughts". well.. .Rome was far away, and Pilate a mere local roman governor, and "religious beliefs" were already important enough (gods' respect etc) to send somebody to death.
Romans are supposedly (WIS says) continuing to support the theory of a material man, since, scnists say, they believed that spirit/life itself disappeared at death. This is not contradictory to the belief in a spiritual part of the man, so Romans were not more "materialistic" regarding man's definition than those believing the "spirit or soul or psyche" could be reinscriptible in some other body - rather the contrary!
Later, the WIS says that Inquisition appeared on the XVth century. It's wrong, its first activities started on XIIIth century.
WIS pretends that Inquisition attacked Leonardo Da Vinci. It's not evident at all. The only problem between christianism and da Vinci is that he had been suspected of homosexuality.
WIS mocks as well Darwin, like if scientology was denying its therories validity.
Most of the chapter presenting the "history of scientology" is designed t heighten some importances (like Buddha!) or diminish some others to einforce some scientology visions.
The same can be observed when studying what scholars studies or texts wer choosen to demonstrate the "religiousness" of the cult in the book "Scientology, theory and practice of a contemporary religion". Very few have
been retained as valid for the viewpoints scn wanted to giv, while scn has paid for 54 scholars...
No surprise, Pr Kent, Pr Beit-Hallahmi, Pr Zablocki weren't retained as valid by the cult.