Dr. Slyke, and I dare to say also St. Augustine (in the quotation referred in S. th. I, q. 51, art. 3, reply to obj. 6)) and St. Thomas Aquinas (as far as he implicitly in his reply endorses Augustine’s view) are not taking into account the crucial verse Luke 3:38, where Adam is called “son of God”.
Interpreting Scripture with Scripture I firstly claim that the term “sons of God” in Genesis 6:1-4 is used in the same sense as Adam is called “son of God” in Luke 3:38.
Accordingly, the term means human persons who are begotten independently of any creature’s decision and therefore cannot be called son of another human person. By contrast, names different from Adam in Luke 3:23-38 are called “son of someone”: “son of David”, “son of Abraham”, son of Noah” etc.
As explained in this Essay I secondly assume that God created the first Image Bearers through transformation of human-like animals into human persons. From this I infer that the “sons of God” of Genesis 6:1-4 were created directly by God through a similar transformation independently of any human father’s decision.
Finally, I note that also angels can be said to be “persons who are begotten independently of any creature’s decision”, since they originate from God independently of any angel’s decision, and therefore this meaning of the term “sons of God” is suitable for angels too, no matter whether they are good or evil, as used for instance in Job 1:6; 2:1. And one could analogously even say that the Second Person of the Trinity is the proper Son of God since He is begotten from the Father independently of whatever decision, even a divine one.
According to Scripture “sons of God” means “persons who are begotten independently of any creature’s decision”. All angels can appropriately be called “sons of God”, but not all characters called “sons of God” in the Bible can appropriately be considered angels. The “sons of God” in Genesis 6:1-4 are undoubtedly human, and it is fitting to consider that they were created directly by God through transformation of human-like animals into persons.
The “sons of God” in Genesis 6:1-4 were not created “at the same time as Adam and Eve” but after “Adam and Eve” sinned (where ‘Adam and Eve’ refer to the ‘first human sinners’). This means according to my explanation “Transmission at generation”, that they shared in the state of original sin with all the bad propensities proper of and were not “holier” than the persons generated through conception after the Fall (as the Genesis narrative confirms).