How can spiritual beings such as angels mate with physical beings like humans?
According to God in the book of
Genesis 6:4 that's precisely what happened.
But the other question is.
As to where is it written that spiritual beings can not mate with physical brings.
Note also this shall happen again, just before the tribulation that is soon to come upon all the world,
That the fallen angels shall come to mate with physical brings. To produce hybrid beings again.
Give this some thought, beings as tall as 35 to 60 feet tall. Let that sink in.
And they will be cannibals, eating human flesh.
As written in the book of
Numbers 13:31-33---"But the men that went up with him said, We be not able to go up against the people; for they are stronger than we"
32 And they brought up an evil report of the land which they had searched unto the children of Israel, saying, The land, through which we have gone to search it, is a land that eateth up the inhabitants thereof; and all the people that we saw in it are men of a great stature.
33 And there we saw the giants, the sons of Anak, which come of the giants: and we were in our own sight as grasshoppers, and so we were in their sight"
Note that these Giants are human hybrids, that came from the fallen angels, the sons of God, back in Genesis 6:4.