If the "sons of God" (Genesis 6:4) were the angels who left their former habitation or first estate – the spiritual realm (Jude 3) – how is it that they could marry the "daughters of men" and have children since it is understood from the words of our Master (Matthew 22:30) that the angels are sexless?

From many scriptures, it is apparent that spirit beings have had the power of materializing as human beings. This materialization is complete, as the following citations indicate, showing that these materialized beings could eat and drink and perform all of the functions of the natural being of the earthly plane of existence: Genesis 18:8, Genesis 32:24, Zechariah 1:9,5, Luke 1:30, Acts 12:7-10. It is evident that the angels in their proper habitation, the spiritual realm, do not possess the powers of reproduction. It would therefore be impossible for them to have angelic progeny. It is only as they materialize as human beings that they could exercise the powers belonging to the race of mankind, and thus bring children into the world of the fleshly nature. Because some of the angels did this without the Divine sanction, they were punished by being restrained in chains of darkness unto the Judgment Day. 

Additional Resource:
Christian Questions Podcast
Episode #1305: How Do Satanic Demons Influence Our World?
Establishing the origin, power and methods of dark spiritual beings
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