Homosexuality is, at worst, a symptom of other causes. As are all the other "causes" Gibbon sites for the decay of society.
Anyone who frequents CureZone should have a pretty good idea how effective targetting symptoms is versus targetting the real causes.
The Roman Empire, for example, took Europe into the Dark Age with the rise of Christianity as the state religion.
The Roman Empire prospered for centuries with homosexuality as an accepted practice. It wasn't homosexuality that brought the Roman Empire down; it was the attack on reason brought about by the rise of zealotry.