The verses never said they could not. GOD allowed it to happen. About contaminating the blood line. The angels cannot see the future, They knew nothings about Jesus birth.
The angels of Genesis 6 were not suppose to come to earth. If they had not done so they wouldn’t have been able to have sex with women…. They received punishment for their actions.
Jude 1:6
And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.
Angels are not stupid, they knew Christ was to be born.
Genesis 3
15 And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.
“There were Nephilim in the earth in those days (i.e. in the days of Noah); and also AFTER THAT,”
So that "after that", i.e. after the Flood, there was a second irruption of these fallen angels, evidently smaller in number and more limited in area, for they were for the most part confined to Canaan, and were in fact known as "the nations of Canaan". It was for the destruction of these, that the sword of Israel was necessary, as the Flood had been before.
As to the date of this second irruption, it was evidently soon after it became known that the seed was to come through Abraham; for, when he came out from Haran (Gen. 12:6) and entered Canaan, the significant fact is stated : "The Canaanite was then (i.e. already) in the land." And in Gen. 14:5 they were already known as "Raphain" and Emim", and had established themselves at Asteroth Karnaim and Shaven Kiriathaim.
Isaiah knew Christ was coming
Isaiah 11:4
Therefore the Lord Himself shall give you a sign: Behold, a virgin shall conceive and bear a Son, and shall call His name Immanuel.
Isaiah 9;6
6 For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace.