According to an Orthodox that replies to someone else in an exchange regarding marital sex, he states "remember the words of Psalm 50" (Psalm 51 in Masoretic-based Bibles):
I was conceived in iniquity and in sins did my mother bear me
We were never meant to have sex before the fall, so at some level no sexual activity could be considered "pure."
After asking him about this view that no sexual activity could be considered pure because we weren't meant to have sex before the fall, he produces an amount of quotes from the early church fathers about the matter:
Saint Gregory of Nyssa, from On the Making of Man:
Now the resurrection promises us nothing else than the restoration of the fallen to their ancient state; for the grace we look for is a certain return to the first life, bringing back again to Paradise him who was cast out from it. If then the life of those restored is closely related to that of the angels, it is clear that the life before the transgression was a kind of angelic life, and hence also our return to the ancient condition of our life is compared to the angels. Yet while, as has been said, there is no marriage among them, the armies of the angels are in countless myriads; for so Daniel declared in his visions: so, in the same way, if there had not come upon us as the result of sin a change for the worse, and removal from equality with the angels, neither should we have needed marriage that we might multiply; but whatever the mode of increase in the angelic nature is (unspeakable and inconceivable by human conjectures, except that it assuredly exists), it would have operated also in the case of men, who were "made a little lower than the angels," to increase mankind to the measure determined by its Maker.
Saint Gregory Palamas, from his homily On the Annunciation:
God sent the archangel to a virgin and made her, who continued a virgin, His mother by means of a salutation alone. If He had been conceived from seed, He would not have been a new man, nor sinless, nor the Saviour of sinners. The flesh's impulse to reproduce is not subject to our minds, which God has appointed to govern us, and is not entirely without sin. That is why David said, "I was shapen in iniquity; and in sin did my mother conceive me" (Ps. 50:5). So if the conception of God had been from seed, He would not have been a new man, nor the author of new life which will never grow old. If He were from the old stock and inherited its sin, He would not have been able to bear within Himself the fullness of the incorruptible Godhead or to make His flesh an inexhaustible source of sanctification, able to wash away even the defilement of our First Parents by its abundant power, and sufficient to sanctify all who came after them.
The same saint, from the homily On the Gospel Reading for the Seventeenth Sunday of Matthew About the Canaanite Woman:
What is the starting point of our coming into the world? Is it not almost the same as for irrational animals? Actually it is worse, because the procreation of animals did not originate from sin, whereas in our case it was disobedience that brought in marriage. That is why we receive regeneration through holy baptism, which cuts away the veil which covers us from our conception. For although marriage, as a concession from God, is blameless, yet our nature still bears the tokens of blameworthy events. For that reason one of our holy theologians [Saint Gregory the Theologian] calls human procreation, "nocturnal, servile, and subject to passion", and before him David said, "I was shapen in iniquity; and in sin did my mother conceive me" (Ps. 50:5)
Saint John Chrysostom, from On Virginity:
When he was created, Adam remained in paradise, and there was no question of marriage. He needed a helper and a helper was provided for him. But even then marriage did not seem to be necessary... Desire for sexual intercourse and conception and the pangs and childbirth and every form of corruption were alien to their soul.
The same saint, from Homilies on Genesis:
Whence, after all, did he come to know that there would be intercourse between man and woman? I mean, the consummation of that intercourse occurred after the Fall; up till that time they were living like angels in paradise and so they were not burning with desire, not assaulted by other passions, not subject to the needs of nature, but on the contrary were created incorruptible and immortal, and on that account at any rate they had no need to wear clothes . . . Consider, I ask you, the transcendence of their blessed condition, how they were superior to all bodily concerns, how they lived on earth as if they were in heaven, and though in fact possessing a body they did not feel the limitations of their bodies. After all, they had no need for shelter or habitation, clothing or anything of that kind . . .
In another place, he says:
“Now Adam knew Eve his wife.” Consider when this happened. After the disobedience, after their loss in the Garden, then it was that the practice of intercourse had its beginning. You see, before their disobedience they followed a life like that of the angels, and there was no mention of intercourse. How could there be, when they were not subject to the needs of the body?
Why did marriage not appear before the disobedience? Why was there no intercourse in Paradise? Why not the pains of childbirth before the curse? Because at that time these things were superfluous. The necessity arose later because of our weakness, as did cities, arts and skills, the wearing of clothes, and all our other numerous needs.
Saint John of Damascus, from An Exact Exposition of the Orthodox Faith:
Carnal men abuse virginity , and the pleasure-loving bring forward the following verse in proof, Cursed be every one that raises not up seed in Israel. But we, made confident by God the Word that was made flesh of the Virgin, answer that virginity was implanted in man's nature from above and in the beginning. For man was formed of virgin soil. From Adam alone was Eve created. In Paradise virginity held sway. Indeed, Divine Scripture tells that both Adam and Eve were naked and were not ashamed. But after their transgression they knew that they were naked, and in their shame they sewed aprons for themselves. And when, after the transgression, Adam heard, dust you are and unto dust shall you return , when death entered into the world by reason of the transgression, then Adam knew Eve his wife, and she conceived and bare seed. So that to prevent the wearing out and destruction of the race by death, marriage was devised that the race of men may be preserved through the procreation of children.
But they will perhaps ask, what then is the meaning of “male and female,” and “Be fruitful and multiply?” In answer we shall say that “Be fruitful and multiply ”does not altogether refer to the multiplying by the marriage connection. For God had power to multiply the race also in different ways, if they kept the precept unbroken to the end. But God, Who knows all things before they have existence, knowing in His foreknowledge that they would fall into transgression in the future and be condemned to death, anticipated this and made “male and female,” and bade them “be fruitful and multiply.” Let us, then, proceed on our way and see the glories of virginity: and this also includes chastity.
Saint Athanasius, from his commentary on the Psalms (specifically Psalm 50:5 in this case):
The original intention of God was for us to generate not by marriage and corruption. But the transgression of the commandment introduced marriage on account of the lawless act of Adam, that is, the rejection of the law given him by God. Therefore all of those born of Adam are “conceived in iniquities,” having fallen under the condemnation of the forefather.
Saint Symeon the New Theologian, from the Ethical Discourses:
There was no one, you see, who was able to save and redeem him. For this very reason, therefore, God the Word Who had made us had pity on us and came down. He became man, not by intercourse and the emission of seed – for the latter are consequences of the Fall – but of the Holy Spirit and Mary the Ever-Virgin.
Saint Maximus the Confessor, from Ad Thalassium:
He [Christ] appeared like the first man Adam in the manner both of his creaturely origin and his birth. The first man received his existence from God and came into being at the very origin of his existence, and was free from corruption and sin – for God did not create either of these. When, however, he sinned by breaking God’s commandment, he was condemned to birth based on sexual passion and sin. Since henceforth constrained his true natural origin within the liability to passions that had accompanied the first sin, as though placing it under a law. Accordingly, there is no human being who is sinless, since everyone is naturally subject to the law of sexual procreation that was introduced after man’s true creaturely origin in consequence of his sin.
Tertullian, from On the Resurrection of the Flesh:
To this discussion, however, our Lord's declaration puts an effectual end: "They shall be," says He, "equal unto the angels." As by not marrying, because of not dying, so, of course, by not having to yield to any like necessity of our bodily state; even as the angels, too, sometimes. Were "equal unto" men, by eating and drinking, and submitting their feet to the washing of the bath-having clothed themselves in human guise, without the loss of their own intrinsic nature.
I could go on, if you want, but I believe this is enough.
My question is Why did the early church fathers thing that sex was a consequence of the Fall? if we think that Adam and Eve did have sex before the fall According to this reply in Biblical Hermeneutics Stack Exchange to Did Adam and Eve not have sex in the Garden of Eden? (granted, they can be wrong as they are not the Church fathers).