Gabriel and the birth of Jesus

Angels had sex with Earth women, says Genesis 6: 1-4

Here’s the Christian story: The angel Gabriel came to Mary and told her she was pregnant. She hadn’t had sex with a man.

How could this be? The Old Testament provides an answer: In Genesis 6: 1-4 it is told how the angels found the daughters of men attractive and had sex with them, and produced powerful men (or heroes, or giants, depending on your preference).

So Jesus can presumably claim descent from God through Gabriel. This is all so logical. I wonder why it isn’t a standard part of the Christmas story?