“And at that point, too, in Buddhist India as in Babylon, in Rome as in our own cities, a man's choice of the woman who is to be, not mother of his children as amongst peasants and primitives, but his own "companion for life", becomes a problem of mentalities. The Ibsen marriage appears, the "higher spiritual affinity" in which both parties are "free"—free, that is, as intelligences, free from the plantlike urge of the blood to continue itself, and it becomes possible for a Shaw to say "that unless Woman repudiates her womanliness, her duty to her husband, to her children, to society, to the law, and to everyone but herself, she cannot emancipate herself." The primary woman, the peasant woman, is mother. The whole vocation towards which she has yearned from childhood is included in that one word. But now emerges the Ibsen woman, the comrade, the heroine of a whole megalopolitan literature from Northern drama to Parisian novel. Instead of children, she has soul-conflicts; marriage is a craft-art for the achievement of "mutual understanding"….
At this level all Civilizations enter upon a stage, which lasts for centuries, of appalling depopulation. The whole pyramid of cultural man vanishes. It crumbles from the summit, first the world-cities, then the provincial forms, and finally the land itself, whose best blood has incontinently poured into the towns, merely to bolster them up awhile. At the last, only the primitive blood remains, alive, but robbed of its strongest and most promising elements. This residue is the Fellah type.
If anything has demonstrated the fact that Causality has nothing to do with history, it is the familiar "decline" of the Classical, which accomplished itself long before the irruption of Germanic migrants. The Imperium enjoyed the completest peace; it was rich and highly developed; it was well organized; and it possessed in its emperors from Nerva to Marcus Aurelius a series of rulers such as the Caesarism of no other Civilization can show. And yet the population dwindled, quickly and wholesale. The desperate marriage-and-children laws of Augustus—amongst them the Lex de maritandis ordinibus, which dismayed Roman society more than the destruction of Varus's legions—the wholesale adoptions, the incessant plantation of soldiers of barbarian origin to fill the depleted country-side, the immense food-charities of Nerva and Trajan for the children of poor parents—nothing availed to check the process.”

Vol. II, Alfred A. Knopf, 1928, pp. 104–06 https://archive.org/stream/Decline-Of-The-West-Oswald-Spengler/Decline_Of_The_West#page/n573/mode/2up/search/depopulation
The Decline of the West (1918, 1923)