As creepy as this sounds (and as offensive as it is to modern sensibilities) there does seem to be a strong tradition for interpreting Genesis 6.1-3 as having to do with angelic beings having sexual relations with female humans.
There is some speculation in the Jewish tradition regarding this text. The author of 1 Enoch writes, “And it came to pass when the children of men had multiplied that in those days were born unto them beautiful and comely daughters. And the angels, the children of heaven, saw and lusted after them, and said to one another: ‘Come, let us choose us wives from among the children of men and beget us children.” (6.1-3a)
In the Testament of Reuben 5.6 (it may have Jewish roots, but it seems likely that it is a Christian document in the form we have it now) the author writes, “Therefore flee fornication, my children, and command your wives and your daughters that they adorn not their heads and their faces; because every woman who acts deceitfully in these things has been reserved for everlasting punishment. For thus they allured the Watched before the flood; and as these continually beheld them, they fell into desire each of the other, and they conceived the act in their mind, and changed themselves into the shape of men, and appeared to them in their congress with their husbands; and the women, having in their minds desire toward their apparitions, gave birth to giants, for the Watchers appeared to them as reaching even unto heaven.”
It is possible that both 2 Pet. 2.4 and Jude 6-7 allude to this interpretation. One may also postulate that 1 Cor. 11.1-16, especially v. 10 is influenced by this legend. When Paul begins to speak of the Spirit-Flesh contrast in his Epistles to the Romans and to the Galatians he does not indicate that his juxtaposition has anything to do with angelic beings corrupting humanity.
In a list compiled by Andrew Louth (Ancient Christian Commentary on Scripture: Old Testament I, 123-124) he quotes Clement of Alexandria and Nemesius of Emesa as also interpreting this passage to be angels