וַֽיְהִי֙ כִּֽי־הֵחֵ֣ל הָֽאָדָ֔ם לָרֹ֖ב עַל־פְּנֵ֣י הָֽאֲדָמָ֑ה וּבָנ֖וֹת יֻלְּד֥וּ לָהֶֽם׃ וַיִּרְא֤וּ בְנֵי־הָֽאֱלֹהִים֙ אֶת־בְּנ֣וֹת הָֽאָדָ֔ם כִּ֥י טֹבֹ֖ת הֵ֑נָּה וַיִּקְח֤וּ לָהֶם֙ נָשִׁ֔ים מִכֹּ֖ל אֲשֶׁ֥ר בָּחָֽרוּ׃ וַיֹּ֣אמֶר יְהֹוָ֗ה לֹֽא־יָד֨וֹן רוּחִ֤י בָֽאָדָם֙ לְעֹלָ֔ם בְּשַׁגַּ֖ם ה֣וּא בָשָׂ֑ר וְהָי֣וּ יָמָ֔יו מֵאָ֥ה וְעֶשְׂרִ֖ים שָׁנָֽה׃ הַנְּפִלִ֞ים הָי֣וּ בָאָ֘רֶץ֮ בַּיָּמִ֣ים הָהֵם֒ וְגַ֣ם אַֽחֲרֵי־כֵ֗ן אֲשֶׁ֨ר יָבֹ֜אוּ בְּנֵ֤י הָֽאֱלֹהִים֙ אֶל־בְּנ֣וֹת הָֽאָדָ֔ם וְיָלְד֖וּ לָהֶ֑ם הֵ֧מָּה הַגִּבֹּרִ֛ים אֲשֶׁ֥ר מֵעוֹלָ֖ם אַנְשֵׁ֥י הַשֵּֽׁם׃
The euhemeristic stance of the church fathers prevailed in the medieval Latin West, where BW was virtually lost.  Even so, Franklin Harkins has shown that “Watchers traditions… found their way into a number of significant medieval scholastic texts,” often only to refute the angelic reading.  In the Glossa ordinaria (1080-1130), the great compendium of patristic and medieval biblical commentary, Reichenau Walafrid Strabo (d. 849) explains Gen 6:1-4 with the Sethite interpretation, overtly stating that giants need not originate from demon intercourse with human women.
The subject found new treatment in the Latin academies of the high middle ages. Scholastics seeking to reconcile angels with the liberal arts worked to understand how incorporeal beings like angels interact with the physical world. Peter Lombard (d. 1160) states that angels can manipulate the physical world by combining “seeds of those things” (semina istarum rerum). Bonaventure (d. 1274) explains that when demons create, they do so through a form of magic, an artifice, not creating through their own nature but by “alien power” (virtute aliena). Bonaventure’s teacher, Alexander of Hales (d. 1245), explains that in this manner incubi “separate the nature with which the child is generated; they assemble it from the secret, innermost places of things and pour it into women, but not from bodies that they assume.” Demons can gather the seeds of human beings and plant them in women, but these seeds do not originate from the demons themselves. 
From this, Thomas Aquinas (d. 1273) built an angelic reading of Gen 6:1-4 which would not necessitate the crossbreeding of demons and humans:
Some say that demons in their assumed bodies are in no way able to procreate; and that ‘the sons of God’ do not signify angels [acting as] incubi, but rather the sons of Seth, and that ‘the daughters of men’ signify those who descended from the race of Cain. But because the contrary is held by many, and [because] what seems true to many cannot be entirely false according to the Philosopher…, it can therefore be said that by their [i.e., the demons’] action generation is completed inasmuch as they can put human semen in a place appropriate to the corresponding matter, just as they can also bring together the seeds of other things in order to bring certain effects to fruition…. So only what is local motion is attributed to them, not however generation itself, the beginning of which is not the power of the demon or of the body assumed by him, but rather the power of that one whose semen it was. Hence the one born is not the demon’s, but rather is the son of the certain human. 
Aquinas performs a balancing act. Without renouncing the traditional Sethite interpretation, he recognizes the popularity of the angelic reading and provides a plausible explanation for it. To explain the origin of giant humans, Aquinas elsewhere suggests that demons may through astrology choose special times to place a man’s seed in a woman’s womb, producing unusually stronger and bigger human offspring.  Although demons cannot of their own nature produce giants in women, demons may appear to bring about the existence of giants.
Aquinas explains how demons can transport human seed from a man to a woman and, through their false artifices, see to it that the offspring are extraordinarily large humans—giants. Such a possibility relies upon the scholastic groundwork laid in previous centuries by Peter Lombard, Alexander of Hales, and Bonaventure. Curiously, in answering the difficult question of whether or not demons may produce offspring with humans, Aquinas recalls Gen 6:1-4. Harkins argues that this “suggests a working knowledge of and even a theological appreciation for the angelic interpretation of this scriptural story as found in I Enoch.” He concludes, “a variety of scholastic works of the twelfth and thirteenth centuries reveal echoes of Watchers traditions, even at precisely those exegetical and theological loci such as Gen 6 where Augustine… sought to mute them.” Harkins even suggests that Augustine’s stance against the Watchers tradition helped preserve it in scholastic circles. 
However, Serenus’ qualification of the angelic reading as “illa vulgi opinio” points to a different means of preservation. The contempt which the church fathers poured on this interpretation betrays its popularity among the lay masses, who easily connected Gen 6:1-4 to Greek myths of supernatural offspring coming from human-divine copulation. Indeed, Aquinas gave the angelic interpretation consideration precisely due to its continued popularity: the angelic tradition “is held by many,” he writes. Aquinas seems to be responding to a popular acceptance of the angelic reading. Whence this popularity? Surely, the masses did not accept the fallen angels tradition merely due to Augustine’s opposition to it. To understand the popular reception of the Watchers tradition, one must look outside the scholastic theological tradition.
The Brut Chronicle and Nicholas of Lyra’s Postillae
Centuries after Augustine, in the high middle ages, well-respected secular storytelling transmitted the motif of giants as descendants of demonic copulation with women, priming the layman to readily accept the angelic reading. Geoffrey of Monmouth’s twelfth-century History of the Kings of Britain provided England with a foundation history, including the myth of Brutus the Trojan who settled Britain and subdued the island’s primeval giants. The stories borrow biblical elements, giving the British a sense of divine election. The chronicle quickly became popular among the learned classes of England, so much so that to this day hundreds of manuscripts survive.
Later medieval chroniclers built upon Geoffrey’s work, including the poet Wace in his Anglo-Norman Brut. In the mid-thirteenth century, a prologue to the Brut emerged to explain the origin of the giants of Britain.  The prologue told the story of princess Albina and her sisters. These women, daughters of a great emperor, had been caught in their plot to kill their husbands. As punishment, their father placed them on a rudderless ship. After suffering many storms, the sisters landed on the uninhabited island of Britain, where they learned to survive. Soon, however, they began desiring intimacy with men, and certain demons took notice. These incubi consorted with the women, and the women gave birth to giants. These were the same giants which, years later, Brutus conquered when he settled the island.
The Anglo-Norman poetic Brut prologue describes the demons’ ability to copulate through language reminiscent of previous theological literature:
Humaine forme perneient These had the ability to take human form
Ovesqe ceo la nature together with human powers 
The poet uses the language of “form” and “nature,” but his simplified depiction of the process conflicts with that described by the scholastic theologians. Aquinas and Alexander of Hales allowed that incubi may gather and move human seed into women, but the poetic prologue states outright that the demons took on human form and the functions associated with human nature.
The later Middle English prose prologue provides more detail on how the Devil procreated with the women, offering an explanation that aligns better with the scholastics:
Whanne the Deuyll… nome bodyes of the eyre & likyng natures shad of men, & come in-to the land of Albyon and lay by the wymmen, and schad tho natures vpon hem, & they conceiued, and after thei broughten forth Geauntes… 
When the Devil… took bodies of the air and liking natures shedding of men [semen], and came into the land of Albyon and lay by the women, and shed those natures upon them, and they conceived, and after they brought forth Giants…
Here the Devil merely takes semen through what Bonaventure calls virtute aliena, gathering and concocting through artificial means human seed which he then places inside the women. Yet, the end result is still a monstrous giant, not a normal human being. Aquinas allows for unusually strong and large humans to come from such a union based on careful astrological timing, but none of this is mentioned in the prologue. Ultimately, the Middle English version gathers ad hoc support from both scholastics and popular imagination.
It bears noting that the Brut chronicle gained a vast readership in the high middle ages. Many manuscripts survive, indicating its wide dissemination, and later chroniclers of British history based their work on the Brut chronicle.  Its translations into Latin and Middle English indicate a multitude of audiences, including noblemen, merchants, and clergy. Its prose and poetic versions point to a range of performance settings.  From this, one may deduce that the motif of incubi producing giants through sexual contact with women, contained in the Albyna prologue, attained a large audience among medieval English laymen and clergy alike. The motif’s re-presentation in secular history and storytelling sounds a playful counterpoint to the scholastic discussions of the abbey.
For this reason, it should not surprise to see later scholastic writers continue to discuss the subject at length. One Franciscan exegete, Nicholas of Lyra (d. 1349), worked at a time when the legend of Albyna had spread among the English population. Lyra’s writings represent the culmination of the Paris school of exegesis. He frequently explained Hebrew words and dialogued with rabbinic commentaries, at times adhering to their suggestions and at other times refuting them. During and shortly after his lifetime, Lyra’s Postillae on the entire Bible spread across the European continent, eventually getting appended to the Glossa ordinaria. English readers celebrated his writings; the marginal glossing in manuscripts of the Wycliffite Bible rely heavily upon Lyra. What did Lyra have to say about Gen 6:2-4?
Lyra offers a few explanations of the lemma videntes filii dei. The first explanation may be the most familiar to his readers: hoc intelliguntur demones incubi—the filii dei are understood as incubi. As support for this stance, Lyra points to Job 1:6, where Satan is among the sons of god. Such incubi intermixed with the human women, from whom were born giants. “But,” Lyra writes, “this explanation does not seem rational… the flood was never a punishment of demons, but only of humans.”  Having discounted this reading, Lyra offers the traditional Jewish interpretation, citing Rashi. Though eloym can refer to judices, the flood judged humanity in general, not merely a specific class of rulers. The better (melius) explanation understands the sons of God to be “those who descended from Seth” and married the female descendants of Cain. This intermarriage with Cain’s descendants went against God’s will, just as God commanded the sons of Israel not to marry Canaanite women. Later, in his comment on Gigantes autem erant, Lyra calls the Gigantes (nĕp̄ilîm) tall and strong, but his description falls short of anything overtly supernatural. In fact, he warns against considering these giants as coming from demons.  Lyra thus upholds the traditional Sethite reading while continuing to bring up the angelic reading.
In refuting the angelic reading, Lyra repeats the words of his patristic forbears, an expected practice from a scholastic exegete. But Lyra may also have felt a need to respond to the angelic reading because of its continued popularity in Western Christendom. Some of his contemporary Christians, influenced by extrabiblical myths such as the Albyna story, could still find in incubi a natural solution to the identity of filii dei. Although the West did not pass down a manuscript tradition of BW, the ideas in this rewritten bible continued to circulate. Harkins suggests these ideas may have been kept alive by Augustine’s polemics against them. But a look at the Brut demonstrates that, in the medieval imaginary, supernatural giants come from incubi and human women. The angelic reading meshed well with Britain’s national legend, so much so that scholastic exegesis of the high middle ages still had to disprove it.
In the Second Temple period, a preoccupation with spiritual beings and an awareness of the mythologies of the Hellenistic world led interpreters to read Gen 6:1-4 as a reference to a cosmological conflict between God, fallen angels, and giants. But as Christian and Jewish interpreters built up the scaffolding of orthodoxy in late antiquity, this expansion of the text fell out of favor. Targum Onkelos replaced bĕnê-hāʾę̆lohîm with bny rbrbyʾ, and Christians understood filii dei as filii seth.  Scholastic exegetes continued to defend the Sethite interpretation through the high middle ages. But even as they derided the angelic reading as vulgar, they credited it as popular. Although BW did not circulate widely throughout the medieval Latin West, the motif of giants originating from the intermixing of demons and human women still attracted a large audience, for it had spread into secular literature. In the Brut chronicle, the biblical etiology for the giants of Canaan inspired the origin story of the giants of Britain. The motif thrived in the high middle ages due to the storytelling of noblemen and merchants, not BW or even the discussions of theologians. In the face of all the rationalists of the middle ages, this popular tale has now come full circle to the modern day, when scholars once again read Gen 6:1-4 as a remnant, a peek at an epic now lost.
 Excerpts of BW remained in Syrian Christian communities, and the Greek version continued to be read and copied in Egyptian monasteries. By the high middle ages, however, Ethiopia had become the main locus of transmission. Under Islamic rule, the Ethiopian church had little contact with the broader Christian world. Reed notes: “it is striking that the Ethiopic collection 1 Enoch was not discovered by the West until 1773, when the Scottish traveler James Bruce brought Geʿez manuscripts to Europe in response to decades upon decades of rumors about the preservation of Enochic books in Ethiopia.” Reed 226-332; quote from 331. Harkins writes that “the text itself of I Enoch seems not to have been transmitted to the medieval West.” F. Harkins, The Fallen Angels Traditions, 178.
 F. Harkins, The Fallen Angels Traditions, 158.
 Peter Lombard, Sent. Bk. II d. 8 c. I (Magistri Petri Lombardi Sententiae in IV Libris Distinctae [ed. Ignatius Brady, O.F.M.; 2 vols.; Grottaferrata: Editiones Collegii S. Bonaventurae Ad Claras Aquas, 1971-1981] I. 365-67; Peter Lombard, The Sentences Book 2: On Creation [trans. Giulio Silano; Toronto: Pontifical Institute of Mediaeval Studies, 2008] 33-35); Alexander of Hales, Glossa in Quatuor Libros Sententiarum Petri Lombardi (ed. Pp. Collegi S. Bonaventurae; vol. II: in librum secundum; Quaracchi: Collegium S. Bonaventurae, 1952) d. 7 n. 30, p. 70; Bonaventure, Commentaria in quatuor libros sententiarum magistri Petri Lombardi, vol. 2: In secundum librum (Quarrachi: Collegium S. Bonaventurae, 1885), d. 7 p. 2 a. 2 qq. I and 2, pp. 196-204; Biblia Latina cum Glossa Ordinaria on Gen 6:1-4. Quoted in F. Harkins, The Fallen Angels Traditions, 163-4.
 Thomas Aquinas, Scriptum in II d. 8 q. I a. 4 sol. 2, trans. F. Harkins, The Fallen Angels Traditions, 174.
 Thomas Aquinas, Quaestiones Disputatae De Potentia Dei, q. 6 a. 8 ad 7.
 F. Harkins, The Fallen Angels Traditions, 178-9. “Furthermore, given the care and approbation with which medieval theologians read Augustine, we can well imagine that the latter’s elaboration of the angelic interpretation of Gen 6 in The City of God—in spite of his rejection of it—had the effect of making information about it available in later centuries. It is quite plausible, then, that Augustine was actually an important source for the echoes of Watchers traditions considered above.”
 G. Brereton, Des grantz gaenz: An Anglo-Norman Poem, xxxii.
 English translation by J. Bliss, An Anglo-Norman Reader, 73.
 F. Brie, The Brut; or the Chronicles of England, (Early English Text Society, 1906), 4, ll. 23-7. I provided Modern English for clarification.
 The prose Brut remains extant in more manuscripts than any other Middle English work, except for the Wycliffite Bible. For more information on its distribution and afterlife, see L. Matheson, The Prose Brut: The Development of a Middle English Chronicle; J. Rajsic, E. Kooper, and D. Hoche (eds.), The Prose Brut and Other Late Medieval Chronicles: Books Have Their Histories: Essays in Honour of Lister M. Matheson.
 Wace’s poetic Brut indicates that the story was meant to be recited at feasts. Bliss, An Anglo-Norman Reader, 61, 75, n. 146. Later prose editions became useful as propaganda.
 Qui dicunt filii dei propter naturam spiritualem. Unde et Job.i.ca.dicitur: Sathan fuisse inter filios dei: et isti in specie humana commiscuerunt se mulieribus: et inde nati sunt gigantes.ut/unde dicitur.infra.gigantes autem erant in terra. Sed hec exposition non videtur rationabilis:quia hic exprimitur causa diluuij quodam inundavit in penam peccatorum. Diluuium autem nunquam fuit in penam demonum:sed tantum hominum.
My transcriptions of Lyra’s Postillae are based on two early printed editions: München, Bayerische Staatsbibliothek M26523 (1471-2) 23v-r and 04287 (1482-3) 23r. In the case of variants, I have picked what I deem the clearer presentation.
 …nec valet quod supra dictum est tales fuisse genitos a demonibus… quia etiam post diluuium in quo delete sunt omnes isti leguntur fuisse gigantes:Deutero.ii.et.iii.ca.et.ij.Regum.xxi.qui tamen fuerunt ab hominibus geniti.
 Augustine, De civitate Dei contra paganos, book XV, chapter 23.