In your explanation of the "Sons of God" you stated that there were fallen angels who took on the form of man. God created angels without the ability to procreate (Matt 22:30). I agree that angels can assume human form, however, they still have to produce genes, DNA and chromosomes. Where do they get this ability if they don't have reproductive organs? I always believed that created beings cannot create.
You ask a probing question, and as you probably guessed, we simply don't have enough information from Scripture to answer your questions definitively. You are correct in assuming that angels and men cannot create something as God Himself can (i.e., ex nihilo), so the demons couldn't produce physical material. Nevertheless, we don't know what angels are capable of accomplishing given their mysterious form.
Clearly, demons can indwell humans and can take a form of human so convincing that men don't even recognize them as anything out of the ordinary. These truths can explain how demons might seduce women, but we still cannot know how they contributed to the reproduction of men. Despite our limited understanding of angel physiology, our teaching concerning the mating of demons and women is based on the context of Genesis 6 and the meaning of the Hebrew words based on their use elsewhere in Scripture.