I have heard it argued that according to the law, the Husband of the marriage is the legal father, regardless of who the biological father is. (Yes, I know, the law often makes no sense.)
What that means is, even if Barry's biological father is American, His LEGAL father is still a Foreigner, and therefore Barry is NOT a "natural citizen."
In all honesty, this is a thorny question that would have to be worked out by the legal experts, but the chances are that the legal system would regard Barack Obama sr as the father for all legal purposes.
Stanley Ann was never legally married to BHO, Sr.
The United States doesn't recognize polygamous marriages.
The "union" between Stanley and Barack Sr. was null and void from the get-go.