I wonder if Edward Gibbon's views are relevant to this.... He, in his rather brilliant work "The Decline and Fall of the Roman Empire", placed a good deal of the blame for Roma's tumble on declining morality and virtues of the Roman citizens. The important thing though is he believed this decline of moral standards rested largely with the ascent of Christianity. It was internal barbarism, just as much as external barbarism that laid the Romans low. Gays, had nothing to do with it - essentially Christianity was something the Roman Empire couldn't exist with - thus a slow death, and eventual fall due to many, many factors often linked to religion.
Same with Steven Runciman - noted scholar of Byzantine history, who believed the blow that really leads to the Eastern Empire's fall came not so much on the Islamic hordes, or the Byzantines own rather complex inner society, but on Christian Crusaders, and the values of western Christianity - not family values, but rather political values, and social outlook based upon Biblical morality and guidelines.
But it's always funny such things are rarely brought up - when conversations turn to such things in such a way, it's often with a pro-Christian sentiment at the detriment of an often unfairly maligned group - gays. Muslims. Abolitionists. Communists. Manichean's. Liberal thinkers. Philosophers. Reformists. Feminists. They come in, things change - WE ALL DIE!!!
That said few experts these days blame a single factor for nation death (even in cases of simplu be conquered) - it's often a mulitude of cause, social, ecnonomic, enviromental, and I've never seen any really plausible argument put forward that changing family structures destroy civilisations - in fact the opposite. Family change and power shifts often went with a culture becoming more civilised. Then families would often change with new religions. New philosophies. New concepts. No theories. Many, many times. Little changes, big changes, but never are these the single cause of a culture going belly up. Nations through history fall, yes, but always with a large number of issues when it's internal. Never just one easy scape goat like - "Gadzooks! This is because we let gays marry" or "damn, premarital sex has brought our awesome culture down on our heads!" or "Criminy, letting our women folk work is the end of us!"
And yes, I am quite confident that the US will either one day be gone, or will be changed from what it is now. At the very least I am sure it won't always be a super power. And I am certain this is a fate that most, if not all, current nations will face. As will those in the future. I'd chuck around words like entropy and that thing about the inherint nature of decline, but that's beside the point - I don't see changing family structures playing a part in this - many other causes though. And I am pretty sure God wont be stepping in to save any of them. Not a one. Because a. He doesn't exist of if he does then b. that's not what God does.