Something else to look at concerning Serpent seed, Cain is not listed in Adams lineage also he is referred to as OF THAT EVIL ONE.
15 And the LORD God took the man, and put him into the garden of Eden to dress it and to keep it.
16 And the LORD God commanded the man, saying, Of every tree of the garden thou mayest freely eat:
Ok So we may freely eat of EVERY tree in the Garden…
17 But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.
This is not a literal tree, just like the tree of life is not a literal tree it is the WORD of God.
16 Unto the woman he said, I will greatly multiply thy sorrow and thy conception; in sorrow thou shalt bring forth children; and thy desire shall be to thy husband, and he shall rule over thee.
So as of right now She had not concieved Adams child, so if it wasnt sex she had with the serpent how was her conception multiplied, if you multiply something by nothing you still have nothing and any woman can tell you that conception is much more painfull than nothing.
4:1 And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the LORD.
Here Adam and Eve concieved,, and then bare cain the first of the twins by different fathers. yes I know it says that she got a man from the lord: 2 things one all men and all things were created by God so yes cain was made by God, and she wasnt very discerning of the Word, she was decieved remember, so she may have just said this in error.
2 And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.
Ok, here is adams 1st son abel, Notice that Cain was given the natural ability to work the ground,, the ground that God had just happened to curse along with the Serpent and his seed strang coincidence.
3 And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD.
Then Cain offers to God fruit from the cursed ground, what a slap in the face.
You say its impossible for a serpent and a human to mate, well you seem to forget that a serpent was NOT a snake, it walked on legs like we do, it was that missing link that scientist have been looking for and will never find. we were closer to the serpent than any other animal, it was subtle, smart enough to talk it probably looked alot like a human.
Again look at this youve heard of a Tigon or a Liger close relatives in a species and they successfully mate and look relatively normal, so Im not sure where you get the idea that it is impossible. you would have to have the actual dna of the serpent to determine such a claim.
Just thought I would add some ideas to this dead thread.