Homosexuality wasn't exclusive to the Roman empire. Empire-wide succession battles every few years were. At the inception of empire, Roman military methods were also unique, superior and unknown to the barbarians. Once the barbarians caught on and adapted, Roman victories became much more costly. Eventually, what used to be hard-fought victories became defeats. Every empire of note has run into this problem - there are ambitious men of ability outside of the empire who want to get their hands on the brass ring. The ones who figure out what to do get to become the successors to existing empires. Rome fell to superior military minds which created something out of nothing, despite Rome's ability to call on centuries of military tradition and accumulated military knowledge.
We don’t really know much about the Germans except for what the Romans and the Greeks wrote about them, and homosexuality wasn’t much mention, so I don’t know how you could make that claim.
I’d welcome it if you had it, but from what I remember from some desultory reading was the Germans didn’t admit that people like that existed.
And actually, if you can say that Rome fell at all, looking at the tremendous drop in central authority, collapsing infrastructure and administration following the transfer of power in the West to the successors, whoever they were and wherever they were as they chose to call themselves Vandals, Goths, Franks and so forth, the result of this dramatic transformation which brought such a qualitative and quantitative change to the Empire, was part of a chain of causation going back to the moral decay described by St. Augustine, St. Martin of Tours and others living at the time.