For all your extensive knowledge of world history, you've failed to give a single example of a primarily exclusive homosexual society at all... much less the established fact that all homosexual societies decline.
Who's being dense?
All you've done is insist that the word homosexuality does not refer to homosexual behavior (which it does) and have therefore created a situation in which there is unlikely to have been a documented homosexual society (only documented bisexual ones which we've already pointed out have actually succeeded and continue to exist).
As for the "historical" terms "bisexuality" and "homosexuality". The word "bisexuality" doesn't even exist in Japanese. While the only native word for homosexuality is "doseiai" (love with the same gender) is precisely what was used to define relationships between men reguardless of their involvement with women on the side. Something tells me, Japanese isn't the only case in which this occurs. Any linguistic distinctions made in Japan between those who are bi and gay have been made in the last 30 years thanks to slang terms absorbed from English.
And if I decide to sleep with a woman: I am having a homosexual relationship. Not a "bisexual" one.
search terms: "historical + homosexuality" (747 results)
"historical + bisexuality" (43 results)
Seems to me "most historians" (even modern ones) are considering homosexuality to be a bit wider than just "gay lifepartners"
Homosexuality - Wikipedia, the free encyclopedia
(an article that mentions Greece quite a lot and includes all of those "bisexual" societies under the "Homosexual" heading)