Gen 3:15  And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.

Adam and Eve knew the passage above!

A “MALE” was to redeem the human race to God.

Gen 4:1  And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the LORD.

Now here is the thing with the passage above, Eve was exited because she finally conceived a MAN and not a WOMAN.

Could have easily been translated as “I have finally gotten a man...”

What I am saying here is that Cain was not their firstborn, but was their firstborn MALE!

From what the verse below points to is that most men had daughters, so there was a female population boom out numbering the males born, hence giving the angels plenty to choose from!

Gen 6:1  And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,

Gen 6:2  That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.