The Roman Empire was growing during a time of peak debauchery. It didn’t start falling until after it became Christian.
That said, only the western half fell during ancient times. The eastern half lasted another thousand years.
Mr. Thomas’ thesis is not supported by the data.
There are several better candidates for the fall of the western Roman Empire: lead poisoning, climate change (natural), decline of academic learning, price controls, wily Goths, etc. But it wasn’t immorality. Any family decline at the time was due more to lead poisoning and/or vows of celibacy.
We had a lead poisoning problem, which is now being fixed now that lead has been taken out of gasoline and paint. But residual lead concentrations are still pretty high in our inner cities, and this may account for some of our crime problems therein.
On the other hand, we are introducing a host of new chemicals and organisms into our bodies on a regular basis. Some of these may account for the growing popularity of homosexuality, metrosexuality, and bronies. A nation of girly-mans might well have a problem maintaining an empire.
Conservatives might want to team up with the environmentalists and non-corporate scientists on this issue.
I remember reading Glubb’s short book as a teenager. IIRC, he divided the “classic” (sic) Roman empire into 2 250-year periods, Republic and then Empire. I think you can get a PDF of it out there, it’s well worth a read and very thought-provoking.
Really it was the western army's disintegration that led to Germans crossing the a defenseless Rhine. Had Theodosius come to terms with one of the usurpers in the west, thing may have just rolled along for the west...