See, she was still a virgin when Jesus was born, because no man had touched her. Angels are not human.

And another thing, Gabriel says, "and he will be called a Son of God." not the Son of God. The Fallen Angels were also called the Sons of God (ie: Anu), so to my mind, Gabriel was telling Mary that Jesus was going to be of the same bloodline as the Fallen Angels/Anunnaki (which is of course, what de Vere has claimed he was as well) and that it would be no secret.