The collapse of both the Greek and Roman empires occurred after their acceptance of homosexuality in their midst. The devestating impacts of homosexuality in our society are soon being felt.
Also, the beginning of classical Greek civilization and the Roman empire occurred after their acceptance of homosexuality.
The Greeks had homosexuality from the start. Achilles and Patroclus in the Iliad prove that. Sappho lived around 600 BC.
The Romans became gayer after conquering the Greeks. But that was well before the height of their power. The main difference is that the Romans had slave boy toys and frowned on sex between male Roman citizens, whereas in Greece male lovers could have equal social status.
Only one Roman emperor didn’t have any known gay lover. That was Claudius. Being straight wasn’t a big help to him, since he had to have his third wife, the notorious Messalina, beheaded for her prolific adultery and related plot to overthrow him, and his next wife Aggrippina Minor allegedly poisoned him. He divorced his first two wives, which for Romans was no big deal.