A platonic interpretation of Christianity leads to a tendency to disparage the body at the expense of the spirit, and taken to its logical conclusions arrives at a position very similar to Buddhism, with its negative view of the "fleshy" appetites in all of their various forms. Chesterton recognised that what Aquinas's chief achievement was to emerge victorious in the fight against this interpretation of Christianity. He also recognised that despite Aquinas's victory it is an interpretation that still lingers in Christianity in temperament if not explicitly expressed doctrine. To illustrate just how powerful this temperament was/is it is interesting to see just how hard it was for marriage to be recognised as a sacrament which imparted Grace by early Christian theologians:
This error then had many forms; but especially, like nearly every error, it had two forms, a fiercer one which was outside the Church and attacking the Church, and a subtler one, which was inside the Church and corrupting the Church. There has never been a time when the Church was not torn between that invasion and that treason........... but in a wilder form outside and a milder form inside. So it was, again, in the seventeenth century, when there was Calvinism outside and Jansenism inside. And so it was in the thirteenth century, when the obvious danger outside was in the revolution of the Albigensians; but the potential danger inside was in the very traditionalism of the Augustinians. For the Augustinians derived only from Augustine, and Augustine derived partly from Plato, and Plato was right, but not quite right. It is a mathematical fact that if a line be not perfectly directed towards a point, it will actually go further away from it as it comes nearer to it. After a thousand years of extension, the miscalculation of Platonism had come very near to Manicheanism.GK Chesterton St Thomas Aquinas.
While St Augustine secured a stable scriptural and rational basis for recognising marriage as a sacrament, his successors raised a barrier which I call the Augustinian impasse. The Augustinian tradition could not move beyond a deeply felt difficulty: how could a sacrament involving sexual intercourse be a means of grace? This led to a paradox, marriage alone of all the sacraments did not impart grace. This is what Peter Lombard taught.People may say that that this attitude was present over a thousand years ago and has no relevance to today but it's important to recognise that the unitive nature of sexual intercourse was only recognised as legitimate in the 1930's, some 19 Centuries after the establishment of Christianity. Part of the reason why the sexual revolution struck with such force in the 1960's was as a reaction excessively repressive attitude toward sexuality which stemmed from an Augustinian understanding of it. But it needs to be understood that this Augustinian interpretation wasn't just limited to sex but extended to the Church's attitude to "fun" in general. Weber, in his magisterial Peasants to Frenchmen, cites how "average" Catholics were driven from the French Church through the efforts of zealous pseudo-Jansenist clergy who pushed the spiritual rigor too far. Wanting everyone to be a saint ends up in making most people sinners.
We can understand why theologians of that era had a problem with sexuality when we set Christian marriage in the context of the eight centuries between St Augustine and the scholastic theologians. The medieval era was marked by the flowering of the religious life for men and women in monasteries and convents, accompanied by a defensive emphasis on asceticism, chastity, virginity, purity. In the eleventh and twelth centuries the struggle to enforce the discipline of celibacy among diocesan clergy was a key element in the papal reform programs and in resisting feudal lay power based in families. St Augustine’s own struggle with purity in his earlier years obviously influenced his writings, but I believe the powerful monastic traditions had greater bearing in maintaining a negative attitude to sexuality in married life.
Marriage was seen as a second-best Christian way of life, a remedy for powerful sexual desire, as St Paul taught “better to marry than to burn” (1 Corinthians 7:9). The key word was concupiscentia, disordered desire. In practice concupiscence was, and is, understood as lustful desire, although Augustine understood it more broadly as all disordered desires and unruly appetites, effects of original sin. Theologians who followed Augustine thus came to the strange conclusion that because of concupiscence in sexual union, marriage is the only sacrament that does not give us grace.
My current understanding of secularisation sees it as a multifactoral problem primarily driven by:
A "tactical withdrawal" of Grace by God due to the corruption of Christian doctrine through two separate pathogens. An overt liberal "laxity" which is easily recognised but still toxic to the faith, and a far more dangerous and yet subtle "orthodox' Christian Buddhism/Manicheanism which corrupts Christianity on the inside under the guise of holiness.