Genesis 1:26-31 "Then God said, 'Let Us make MAN in Our image, according to Our likeness; let them have dominion over the fish of the sea, over the birds of the air, over the cattle, over all the earth and over every creeping thing that creeps on the earth.'
So God created man in His own image; in the image of God He created him; male and female He created them. ... Then God saw every thing that He had made, and indeed it was very good. So the evening and the morning were the SIXTH day (Hebrew 'YOM' meaning period of time. It is also translated 'season', 'time', 'while', 'age', 'year' elsewhere in the Bible)."
Genesis 2:1-25 "... And on the SEVENTH day (Hebrew 'YOM' again) God ended His work which He had done ... And the Lord formed man of the dust of the ground ... and He took one of his (Adam's) ribs ... He made into a woman.. Therefore a man shall leave his FATHER and MOTHER and be joined to his wife and they shall become one flesh."
Genesis 3:1-24 "Now the serpent was more cunning ... he said to the woman 'You will not surely die." ... she took of the fruit and ate ... she gave to her husband .. and they knew that they were naked .. and the heard the sound of the Lord God walking in the garden ... "
Genesis 4:1-5 "Now Adam knew Eve his wife, and she concieved and bore Cain ... Then Cain wen out from the presence of the Lord and dwelt in the land of Nod on the east of Eden .. and Cain knew His wife .. and he (Cain) built a city... And Adam knew His wife again, and she bore a son and named him Seth"
Genesis 6:1-4 "Now it came to pass, when MEN began to multiply on the face of the earth, and daughters were born to them, that the sons of GOD saw the daughters of MEN, that they were beautiful; and took wives for themselves of all whom they chose."
Here we have men and women created on the SIXTH period of time, and Adam and Eve created especially on the SEVENTH period of time. This lines up with archeology that has discovered bones of primiative societies that are thousands of years older than when 'civilisation' started, and in different locations. Men and women of the Sixth period had dominion over the WHOLE earth, Adam and Eve lived in the garden of Eden.
Adam knew that people had mothers and fathers even though he had no children yet - and he knew of marriage.
Cain married and built a city when he was cast from the Lord (who was still with Adam and Eve and later their family), even though Seth hadn't been born yet, nor any other siblings.
The sons of God were the decendants of Adam and Eve who still dwelt in the presence of God, the daughters of men were the decendants of those people created on the Sixth period of time and who were all over the earth.
And if you follow down to the Flood. Noah and his sons and their families were saved, along with a limited number of animals in the Ark. But interestingly Noah was the last of those people who lived for long periods of time, around 900 years. He was also a decendant of Adam. Noah can be seen as the last of the people who had dwelt in the presence of the Lord and could be called a 'son of God'.
God created each animal 'after its own kind'. Any cross breeding of species can only work with closely related species (lion-tiger, horse-donkey) and the result is ALWAYS sterile. If man could cross with the angelic kind the result would be sterile, following the law for all things.
Also note that when Jesus had been resurrected He appeared in the FORM of a MAN. But He could appear and disappear at will, He could walk through walls; He was of a likeness of the angels. Just because the angels looked like men doesn't mean they could interbreed with them. Please note that Lot knew them as Angels, it doesn't mean that everyone else did. If the people of the city knew them only as men by their looks, surely they would say 'the MEN'.