Why has this never been a problem for previous civilizations? If yo uthink America is bad... wait until you see China in about 20-30 years.
This has been a problem before. Look at the fall of the Roman Empire, for example.
From The Story of Civilization by Will Durant (1946):
...“Prostitution flourished. Homosexualism was stimulated by contact with Greece and Asia; many rich men paid a talent ($3,600) for a male favorite; Cato complained that a pretty boy cost more than a farm. But women did not yield the field to these Greek and Syrian invaders. They took eagerly to all those supports of beauty that wealth now put within their reach. Cosmetics became a necessity, and caustic soap imported from Gaul tinged graying hair into auburn locks.
“Women won the free administration of their dowries, divorced their husbands or occasionally poisoned them, and doubted the wisdom of bearing children in an age of urban congestion and imperialistic wars. Already by 160 Cato and Polybius had noted a decline of population and the inability of the state to raise such armies as had risen to meet Hannibal. The new generation, having inherited world mastery, had not time or inclination to defend it; that readiness for war which had characterized the Roman landowner disappeared now that ownership was being concentrated in a few families and a proletariat without stake in the country filled the slums of Rome. Men became brave by proxy; they crowded the amphitheater to see bloody games and hired gladiators to fight before them at their banquets.” (Will Durrant, The Story of Civilization, Caesar and Christ, pp. 89–90)
[...]
“What had caused this fall in population? Above all, family limitation. Practiced first by the educated classes, it had not seeped down to a proletariat named for its fertility; by a.d. 100 it had reached the agricultural classes. … Though branded as a crime, infanticide flourished as poverty grew. Sexual excesses may have reduced human fertility; the avoidance or deferment of marriage had a like effect, and the making of eunuchs increased as Oriental customs flowed in to the West. …
“The rapidly breeding Germans could not understand the classic culture, did not accept it, did not transmit it; the rapidly breeding Orientals were mostly of a mind to destroy that culture; the Romans, possessing it, sacrificed it to the comforts of sterility. Rome was conquered not by barbarian invasion from without, but by barbarian multiplication within.
“Moral decay contributed to the dissolution. … Men had now, in the middle and upper classes, the means to yield to temptation, and only expediency to restrain them. Urban congestion multiplied contacts and frustrated surveillance; immigration brought together a hundred cultures whose differences rubbed themselves out into indifference. Moral and esthetic standards were lowered by the magnetism of the mass; and sex ran riot in freedom while political liberty decayed.” (Durant, pp. 666–67.)