Let me see if I can clarify my stance, though I am not completely dogmatic concerning it:
In Genesis 6, it states, " That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose."(vs 2). Now, when Cain slew his brother Abel, God thrusted him out from amongst his family, placing a seal upon him, that if any killed him, greater woe would come upon him(sevenfold). He went into the land of Nod, met his wife and "knew" her and she birthed Encoh, and Cain named the city Enoch, after his first born. IOW, Cain built his own dweling city to dwell in.
Eve then gave birth to Seth, with meant "compensation" or "substitute". IOW, he took over Cain's place in Adam's family. Cain was akin to those angels who fell, who left their first estate. He was like the angels who were bound in chains in the darkness, being thrust out from his Light source.
So, it appears to me, that Cain's seed and Seth's seed, the daughters of men and the sons of God could be the offsprings of Cain and Seth having "relations". Again, just my two pennies...