That's a good point, but theologians who say, "We don’t know how divine beings could produce offspring with humans, so it didn’t happen,” are committing a logical fallacy: lack of understanding is not proof of impossibility (argumentum ad ignorantiam).
Theologians who take the Genesis 6 account seriously from a supernatural perspective have proposed several explanations:
• Some believe the sons of God temporarily assumed physical, human-like bodies—just as angels do in Genesis 18–19—making procreation possible.• Others suggest fallen angels possessed human men, corrupting the human line without direct angelic reproduction.
• Another view sees the union as involving forbidden knowledge—occult or genetic corruption—as described in 1 Enoch.
• And some simply accept the event as literal but mysterious, holding that ancient texts report spiritual realities without needing to satisfy modern biological frameworks.
James Blish, in his novel "Black Easter" (originally serialized in "If" magazine in 1967 under the title "Faust Aleph-Null"), offers an elegant and clever explanation: That succubi (presenting-as-female demons) "couple" with human males and retain their semen. Then, after transforming themselves into incubi (presenting-as-male demons), use it to impregnate (willing) human females.
Regards,