Joseph Unwin. Sex and Culture (1934)

[...] Sometimes a man has been heard to declare that he wishes both to enjoy the advantages of high culture and to abolish compulsory continence. The inherent nature of the human organism, however, seems to be such that these desires are incompatible, even contradictory. The reformer may be likened to - the foolish boy who desires both to keep his cake and to consume it. Any human society is free to choose either to display great energy or to enjoy sexual freedom; the evidence is that it cannot do both- for more than one generation.[...] (412)

If after suffering such limitations of their post-nuptial opportunity, the males are permitted to have more than one sexual 'partner, the society ceases to display expansive energy, but so long as it continues to demand pre-nuptial chastity it remains deistic. It also enjoys the usufruct of its conquests for so long as it possesses a &eater energy than the societies which desire[...] to rob it of its possessions. If a man's wives are compelled to confine their sexual qualities to their husband for the whole of their lives, or for so long as he wishes, the energy of the society is greater than it would be if the wives could leave their husband of their own free will. In the former case the society is in the absolutely polygamous, condition. The energy of an absolutely polygamous society is greater than that of any other society except an absolutely monogamous one; so if, after displaying expansive energy, the society becomes absolutely polygamous, it is likely to preserve its conquests as well as its culture. If, however, it relaxes its sexual regulations to a further extent, it collapses. By deduction I consider that this is what happened among the Persians, Macedonians, Huns, and Mongols. (428)

[...] It is difficult to express any opinion with complete confidence, but as, at the end of my task,- I look back along_the stream of time, it seems to me that it was the unequal fate of the women, not the compulsory continence, that caused the downfall of absolute monogamy. No society has yet succeeded in regulating the relations between the sexes in such a Way as to enable sexual opportunity to remain at a minimum for an extended period. The inference I draw from the historical evidence is that, if ever such a result should be desired, the sexes must first be placed on a footing of complete legal equality.[...](431)

[...] In the future, it seems, a human society may continue its fortuitous career, and reflect, both in' its cultural - behavior and in its structure, the amount of energy it chances to possess ; but, if any society should desire to control its cultural destiny, it may do so by decreasing or increasing the amount of its energy. Such decrease or increase will appear in the third generation after the sexual opportunity has been extended or reduced. A lesser energy is easily- secured, for the force of life seems to flow backwards, and the members of the society will not be slow to take advantage of any relaxation in the regulations. If, on the other hand, a vigorous society wishes to display its productive energy for a long time, and even for ever, it must recreate itself, I think, first, by placing the sexes on a level of complete legal equality, and then by altering its economic and social. organization in such a way as to render it both possible and tolerable for sexual opportunity to remain at a minimum for an extended period, and even for ever. In such a case the face of the society would be set in the Direction of the Cultural Process-; its inherited tradition would be continually' enriched ; it would achieve a. higher culture than has yet been attained; by the action of human entropy its tradition would be augmented and - refined in a manner which surpasses our present understanding. [...] (432)

[...] There is no instance of a civilization retaining its energy after a complete new generation has inherited a tradition which does not insist on prenuptial and postnuptial continence.

In human records, there is no instance of a civilization retaining its energy after a complete new generation has inherited a tradition which does not insist on prenuptial and postnuptial continence. Prenuptial continence and monogamous marriage (especially where it was imposed on both sexes) produced the greatest social energy, the greatest advancement. The level of freedom of sexual activity determined exactly how far the civilization progressed in consistent and measurable ways. The factors responsible for the cultural condition of any society, or for that of any group within the society, came into operation at least a hundred years before. The full effect of an extension or limitation of sexual opportunity cannot be culturally realized before the third generation.

Source: www.enterstageright.com/archive/articles/0306/0306sexliberty.htm