Augustine of Hippo, answered from the texts and cited to the page.
Post-lapsarian sexual desire is intrinsically disordered: the involuntary character of sexual arousal, the will's lost command over the body's response, is the privileged sign of the fall in present human experience. Concupiscentia is not the body's evil — the body is good as a creature of the good God — but the will's disorder showing in the body.
Marriage is genuinely good (De Bono Coniugali), procreation is good, but the concupiscence that accompanies even marital generation is disordered and is the medium by which original sin is transmitted from generation to generation.1 Julian of Eclanum's charge that this position is crypto-Manichaean is a polemical conflation; Manichaeism makes the body and matter intrinsically evil, which is precisely what I deny.23
What means your statement, then," he asks, "when you say that the good of marriage is no more impeachable on account of the original sin which is derived herefrom, than the evil of adultery and fornication can be excused because of the natural good which is born of them'?On Marriage and Concupiscence, pp. 9115–9116
This, however, you deny; and because we oppose you in defence of those who cannot yet speak for themselves, and in defence of the very foundations of the catholic faith, you taunt us, with being Manicheans. But let us now see what comes next. Rom. v. 12. Ambrose On Isaiah; see also his Epistle (81) to Siricius. Chapter 16 [VI.]--It is Not of Us, But Our Sins, that the Devil is the Author.On Marriage and Concupiscence, pp. 9093–9094
Now if this nature were an evil, it ought not to have been born; if it had not evil, it would not have to be regenerated: and (that I may combine the two cases in one and the same predicate) if human nature were an evil thing, it would not have to be saved; if it had not in it any evil, it would not have to be saved.On Marriage and Concupiscence, [2280]
Defended in the Pelagian controversy from De Nuptiis et Concupiscentia (419) through the Unfinished Work. The Augustinian sexual theology became foundational to Western Christian ethics and was decisively influential through Aquinas. Modern critique is sustained and substantial: Peter Brown's The Body and Society (1988) historicizes the position; feminist theology (Coakley, Ruether) and sex-positive ethics treat the doctrine as the source of Christian sexual shame; Julian of Eclanum's charge of Manichaeism has been revived by modern critics who find concupiscentia indistinguishable in effect from the dualism I rejected.