The Bible does not say the serpent had a sexual union with Eve. The Bible plainly teaches that the serpent lied and tricked Eve into eating the forbidden fruit.

The account of Adam and Eve and the serpent in the Garden of  Eden is found in Genesis, chapters 2 and 3.  "The LORD God took the man and put him in the Garden of Eden to work it and take care of it.  And the LORD God commanded the man, "You are free to eat from any tree in the garden; but you must not eat from the tree of the knowledge of good and evil, for when you eat of it you will surely die."  (Gen. 2:15-17)  After Adam had been in the Garden for a time, the Lord made a "suitable helper for him," a woman made from Adam's rib.  (Gen. 2:20-22)

Adam and Eve must have been very happy in the Garden of Eden, a paradise where all their needs were taken care of and where they had perfect communion with God.  But "the serpent was more crafty than any of the wild animals the LORD God had made.  He said to the woman, 'Did God really say, You must not eat from any tree in the garden?' "  After Eve responded that God had told them they must not eat of the one tree, or else they would die, the serpent told his first great lie, "You will not surely die."  (Gen. 3:2-4)

Eve was deceived (or tricked); she ate the fruit, and gave some to Adam, who also ate it.  Now they had to pay the consequences.  In verses 17 to 19 of chapter 3, God told Adam he would now have a hard life, and in the end he would die.  But God also pronounced judgment upon the serpent.  One of the things God told the serpent was that, "I will put enmity between you and the woman, and between your offspring and hers; he will crush your head, and you will strike his heel."  (Gen. 3:15)  This was the first hint that God would eventually send a Savior and that the serpent would be destroyed.

But who was this serpent who could talk?  In Revelation 20:2, pertaining to the last days, we read, "

[An angel] seized the dragon, that ancient serpent, who is the devil, or Satan, and bound him for a thousand years."  (Italics added)   In John 8:44, our Lord Jesus accused the Jewish people who were questioning him, "You belong to your father, the devil…. When he lies, he speaks his native language, for he is a liar and the father of lies."  Satan, the "father of lies," spoke through the serpent in the Garden, in order to deceive Eve and to cause Adam to sin.

Adam's sin and imperfection was inherited by all who were  born into the world, and they were condemned to die.  But God sent His Son, Jesus Christ, as a perfect, sinless human being to die in place of Adam.  Jesus' death was a ransom to buy back Adam and his race from sin and its consequences, death.   In God's due time all will come back from the grave.   "For since death came through a man, the resurrection of the dead comes also through a man.  For as in Adam all die, so in Christ all will be made alive."  (I Cor. 15:21, 22)

As for Cain, he was Adam and Eve's first child.  (Genesis 4:1)  He became the first murderer, but he will rise from the dead; and if, after a period of judgment, he accepts God's righteous way, he will also gain perfect human life.  "Do not marvel at this; for an hour is coming, in which all who are in the tombs will hear His voice, and will come forth; those who did the good deeds to a resurrection of life, those who committed the evil deeds to a resurrection of judgment."  (John 5:28, 29 – NASB)