“Well, apparently they are capable of impregnating human women (Genesis 6:4), so their sperm must have contained genetic material suitable with the genetic material present in human ova.”
Or they are just able to shape shift.
Occam's Razor (the principle of parsimony) suggests that my explanation - which requires fewer assumptions - is to be preferred over your explanation (which demands "Star Trek"-level fantasy suppositions).
Regards,