How the "sons of God" (aka the fallen angels) will be mixing with the seed of men (not the daughters men) would be through the mark of the beast.
This would change you to the point you can't be saved.
Like Esau who sold his birthright for some food and couldn't find repentance even with tears: people will sell their soul, and the right to the earth, by taking the mark of the beast to be able to eat.
They will not be able to repent.
People have had their DNA changed before by bone marrow transplants.
This happened to a man in Germany who did this and the DNA in his blood and semen changed to that of the donor.
