Pfluegermeister at TFN notes that in the November 1, 1981 revised second draft of ROTJ, Luke delivers this line to the Emperor:
"Never! Never will I turn to the dark side. You have failed, your highness. I am a Jedi as my father was before me, and as his father was before him."
The November 1 revised second-draft script was one where Lucas took Kasdan's typewritten second draft and added his own handwritten revisions. Thus, Lucas himself was responsible for this line.... which entirely excludes the possibility of a virgin birth for Anakin.
The idea that Anakin's own father (whoever he might be) was a Jedi certainly reinforces the idea that GL was already thinking about Force potential in genetic terms. On the other hand, it makes the story that Obi-Wan had to "discover" Anakin and recruit him into the Jedi Order (as Ben tells Luke in Kasdan's draft) a bit strange.