[TJJJ]

Furthermore, if that was possible then why is it not possible now?

Why don't we see humans and devils mating now?

There were some definitive things that happened at the curse.

1-Adam had to contend with weeds to grow garden, he was cast out of the garden of eden.
2-Eve would now have to have pain in childbearing as opposed to no pain, she would be subject to her husband.
3-The serpent would now have to crawl instead of .....?

We do not see any other changes physically except that the serpent, who was the serpent before the change, lost his means of motivation other than crawling.

If serpent seed doctrine was true then they should still be able to mate.