St. Augustine writes in Book 15, Ch. 23 of the City of God:
"In the third book of this work (c. 5) we made a passing reference to this question, but did not decide whether angels, inasmuch as they are spirits, could have bodily intercourse with women. For it is written, Who makes His angels spirits, that is, He makes those who are by nature spirits His angels by appointing them to the duty of bearing His messages. For the Greek word ἄγγελος, which in Latin appears as angelus, means a messenger. But whether the Psalmist speaks of their bodies when he adds, and His ministers a flaming fire, or means that God's ministers ought to blaze with love as with a spiritual fire, is doubtful. However, the same trustworthy Scripture testifies that angels have appeared to men in such bodies as could not only be seen, but also touched. There is, too, a very general rumor, which many have verified by their own experience, or which trustworthy persons who have heard the experience of others corroborate, that sylvans and fauns, who are commonly called incubi, had often made wicked assaults upon women, and satisfied their lust upon them; and that certain devils, called Duses by the Gauls, are constantly attempting and effecting this impurity is so generally affirmed, that it were impudent to deny it. From these assertions, indeed, I dare not determine whether there be some spirits embodied in an aerial substance (for this element, even when agitated by a fan, is sensibly felt by the body), and who are capable of lust and of mingling sensibly with women; but certainly I could by no means believe that God's holy angels could at that time have so fallen, nor can I think that it is of them the Apostle Peter said, For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment. 2 Peter 2:4 I think he rather speaks of these who first apostatized from God, along with their chief the devil, who enviously deceived the first man under the form of a serpent. But the same holy Scripture affords the most ample testimony that even godly men have been called angels; for of John it is written: Behold, I send my messenger (angel) before Your face, who shall prepare Your way. Mark 1:2 And the prophet Malachi, by a peculiar grace specially communicated to him, was called an angel. Malachi 2:7."
And on the valid elements of Sacred Tradition in the Book of Enoch, he admits:
"Let us omit, then, the fables of those scriptures which are called apocryphal, because their obscure origin was unknown to the fathers from whom the authority of the true Scriptures has been transmitted to us by a most certain and well-ascertained succession. For though there is some truth in these apocryphal writings, yet they contain so many false statements, that they have no canonical authority. We cannot deny that Enoch, the seventh from Adam, left some divine writings, for this is asserted by the Apostle Jude in his canonical epistle. But it is not without reason that these writings have no place in that canon of Scripture which was preserved in the temple of the Hebrew people by the diligence of successive priests; for their antiquity brought them under suspicion, and it was impossible to ascertain whether these were his genuine writings, and they were not brought forward as genuine by the persons who were found to have carefully preserved the canonical books by a successive transmission. So that the writings which are produced under his name, and which contain these fables about the giants, saying that their fathers were not men, are properly judged by prudent men to be not genuine; just as many writings are produced by heretics under the names both of other prophets, and more recently, under the names of the apostles, all of which, after careful examination, have been set apart from canonical authority under the title of Apocrypha. There is therefore no doubt that, according to the Hebrew and Christian canonical Scriptures, there were many giants before the deluge, and that these were citizens of the earthly society of men, and that the sons of God, who were according to the flesh the sons of Seth, sunk into this community when they forsook righteousness. Nor need we wonder that giants should be born even from these. For all of their children were not giants; but there were more then than in the remaining periods since the deluge. And it pleased the Creator to produce them, that it might thus be demonstrated that neither beauty, nor yet size and strength, are of much moment to the wise man, whose blessedness lies in spiritual and immortal blessings, in far better and more enduring gifts, in the good things that are the peculiar property of the good, and are not shared by good and bad alike. It is this which another prophet confirms when he says, These were the giants, famous from the beginning, that were of so great stature, and so expert in war. Those did not the Lord choose, neither gave He the way of knowledge unto them; but they were destroyed because they had no wisdom, and perished through their own foolishness."