Really, so who were the sons of god that cross bred with humans and wrecked the planet just before the flood of Noah.
They were angels; fallen angels:
Job 1:6 (MKJV):
(6) And a day came when the sons of God came to present themselves before Jehovah. And Satan also came among them.
Job 38:4-7 (WEB):
(4) “Where were you when I laid the foundations of the earth? Declare, if you have understanding.
(5) Who determined its measures, if you know? Or who stretched the line on it?
(6) Whereupon were its foundations fastened? Or who laid its cornerstone,
(7) when the morning stars sang together, and all the sons of God shouted for joy?
Jude 1:6 (ASV):
(6) And angels that kept not their own principality, but left their proper habitation, he hath kept in everlasting bonds under darkness unto the judgment of the great day.
The Greek word that's translated as habitation in Jude 1:6 is
oiketerion, which is only used in this verse and in 2 Corinthians 5:2 -
2 Corinthians 5:1-4 (WEB):
(1) For we know that if the earthly house of our tent is dissolved, we have a building from God, a house not made with hands, eternal, in the heavens.
(2) For most certainly in this we groan, longing to be clothed with our habitation which is from heaven;
(3) if so be that being clothed we will not be found naked.
(4) For indeed we who are in this tent do groan, being burdened; not that we desire to be unclothed, but that we desire to be clothed, that what is mortal may be swallowed up by life.
Paul is saying that we are clothed, or live in, a human body, but after we die and we are resurrected God will give us (Christians) an immortal spiritual body to live in, and he uses the same word - habitation (
oiketerion). It was the spiritual bodies that the angels had that they left and took on human-like form, and then forcibly took and mated with which ever women that they fancied. Their different DNA led to offspring who were superior to humans - the Nephilim.
Who is going to try to mix with the seed of men in the last days.
After the flood these (or other angels) once again mated with women, resulting in more Nephilim (e.g. Goliath). Genesis 6:4 (WEB):
(4) The Nephilim were in the earth in those days, and also after that, when God’s sons came in to men’s daughters and had children with them. Those were the mighty men who were of old, men of renown.
Daniel 2:43 (WEB):
(43) Whereas you saw the iron mixed with miry clay, they will mingle themselves with the seed of men; but they won’t cling to one another, even as iron does not mix with clay.
This is an allusion to the fallen angels trying once again in the last days to take on human form. Who mingles with the seed of men? It has to be beings other than human. We hear of how people are being abducted by "extra-terrestial beings" (angels/demons) and having sexual experiments performed on them. That is most likely because they are working on taking on human form again, but in such a way that they do not produce giants but people who can merge in less noticably.
Unless both are biological in nature there will be no thought of cross breeding.
Yes, they need to be physical animals with close matching DNA, in particular the same number of chromsomes.