It is no secret, for those whom it is no secret, that Augustine believed that original sin was a genetic material stuff that was propagated in and among the human mass 
through the lust of sexual intercourse. Indeed, some of this, no doubt, was developed in the context of his Manichean background; but also, Augustine believed that the passions themselves were ultimately representative of the very base of sin, or what he identified as concupiscence (self-love). He wasn’t the only one who believed this, there were many others, following, like Ambrose, and later Peter Lombard, who affirmed the same in regard to the relationship between post-fall sexual relationships and the propagation of original sin sired in that act. Here is Lombard in his Sentences giving his own view on this, followed by prooftexts from both Ambrose and Augustine.
Chapter 4 (209)
HE SHOWS THE CAUSE OF THE CORRUPTION OF THE FLESH, FROM WHICH SIN OCCURS IN THE SOUL. For the flesh became corrupt in Adam through sin. Before sin, a man and a woman could join together without the incentive of lust and the burning of concupiscence, and there was a marriage bed without stain; but after sin, there cannot be carnal joining without lustful concupiscence, which is always a flaw, and even a fault, unless it is excused by the goods of marriage. And so it is in concupiscence and lust that the flesh is conceived which is to be formed into the body of the offspring. And so the flesh itself, which is conceived in vicious concupiscence, is polluted and corrupted. From contact with such flesh, the soul, as it is infused, derives the stain by which it is polluted and becomes guilty, that is, the vice of concupiscence, which is original sin.
- THAT BECAUSE OF THE CORRUPTION OF THE FLESH, WHICH IS THE CAUSE OF SIN, SIN IS SAID TO BE IN THE FLESH. And that is why sin is said to be in the flesh. And so the flesh which is sown in the concupiscence of lust has neither guilt nor the action of guilt, but its cause. Therefore, in that which is sown, there is corruption; in that which is born by concupiscence, there is vice.
- Hence Ambrose, on the words of the Apostle, says as follows: “How does sin live in the flesh, since it is not a substance, but a privation of the good? In this way: the body of the first man was corrupted through sin, and that corruption remains in the body through the nature of the offense, preserving the force of the divine sentence promulgated against Adam, by association with whom the soul is stained by sin. And so it is because the cause of the deed remains that sin is said to live in the flesh.” This is the law of the flesh.—The same: “Sin does not live in the spirit, but in the flesh, because the cause of sin is from the flesh, not from the soul; because the flesh is, from its origin, of the flesh of sin, and through its transmission all flesh becomes [flesh] of sin.” But the soul is not transmitted and so it does not have the cause of sin in it.
- Augustine too, in a sermon On the Words of the Apostle, shows that the soul contracts sin from the flesh; he says: “The vice of concupiscence is what the soul contracted, but from the flesh. For human nature was not first established with vice by God’s work, but it was wounded by vice coming from the choice of the will of the first humans,” so that there is not good in the flesh, but vice, by which the soul is corrupted.[1]
A truly unbiblical development and accretion, but one that Augustine, and those following, felt needed to be pressed in order to keep the heretical teachings of Pelagius, and the Pelagians at bay. This is what happens when an imbalance is presented into the mix of theological development, particularly as that obtains in the heat of polemical relationships. Not to mention, by time the Augustinian and Pelagius disputation occurred, theological matters had also become a deeply political aggression.
Clearly, from Scripture, the Apostle encourages sexual intercourse (not to mention the epistle to the Hebrews) as a duty, and yet a pleasure, to be had in the marriage bond between a man and a woman.
Now concerning the things about which you wrote, it is good for a man not to touch a woman. 2 But because of immoralities, each man is to have his own wife, and each woman is to have her own husband. 3 The husband must fulfill his duty to his wife, and likewise also the wife to her husband. 4 The wife does not have authority over her own body, but the husband does; and likewise also the husband does not have authority over his own body, but the wife does. 5 Stop depriving one another, except by agreement for a time, so that you may devote yourselves to prayer, and come together again so that Satan will not tempt you because of your lack of self-control. 6 But this I say by way of concession, not of command. 7 Yet I wish that all men were even as I myself am. However, each man has his own gift from God, one in this manner, and another in that.[2]
Not simply as an act for reproduction, as Augustine would press. But indeed, as an act of undefiled and pure intimacy; i.e., the two become one in the ‘flesh’ in the communal bond of the love of the Holy Spirit. Latterly, Bernard of Clairvaux, Martin Luther, Puritans like Richard Sibbes et al. would pick up on what came to be called ‘Marriage Mysticism,’ in regard to the relationship between Christ and His Bride, the Church. In other words, the intimacy envisaged by the act of sexual penetration in the bonds of holy matrimony were so (and are so) sanctified, that it could be the symbol of the intimacy that Christ and His Bride have within the bond of God’s Holy Life as Father, Son, and Holy Spirit (whose life itself is known to be perichoretic and interpenetrative in a Subject-in-Being fellowshipping relationship) (see Eph. 5:18ff). All of this to underscore that sex within marriage coram Deo is a peak event in the human experience in regard to attesting to the very triune life of God itself; inclusive of God’s life with us (Immanuel) and in us in Christ.
Augustine, Ambrose, Lombard et al. couldn’t get everything right.
[1] Peter Lombard, The Sentences: Book 2 On Creation: Distinction XXXI, trans. by Giulio Silano (Toronto, Ontario, Canada: Pontifical Institute of Mediaeval Studies, 2012), 154–55.
[2] I Corinthians 7:1–7, NASB95.