There isn't a credible historian anywhere who attributes gay marriage or the acceptance of homosexuality as a cause of the fall of the Roman Empire.
There isn't a credible historian that doesn't recognize that the Roman
Empire was the representation of the decline of Rome.
Part and parcel of that decline was the normalization of abnormality; debauchery, deviancy, hedonism... subjectivism, being set above the essential objective requirements of any viable culture.
That you can't find the strength of character to recognize that homosexuality is the embodiment of precisely that, is merely you demonstrating yourself to be a classic representation of the very same problem that inevitably sacked the Romans. Thus such is you represented as a classic example of "THE MODERN PROBLEM" expressed in the same old terms that have always defined evil; as such is the definition of evil.
Evil: profound immorality, wickedness, and depravity; profoundly immoral and malevolent.