That is a good article. In all likelihood, what the Framers intended by the phrase “natural born citizen” was the equivalent of what English law in the late 18th century meant by the phrase “natural born subject,” which included (with some exceptions) persons born in England and persons born outside of England with one or two English parents. A person in a “natural born citizen” of the United States if born in the United States of foreign parents (diplomats excepted). Assuming Obama was born in Kenya, his mother was an American citizen, and he would fall in the understanding of “natural born citizen” at the time those words were written. I could be wrong, and I understand the arguments both ways, but on balance that is how I think it will play out.
There are many arguments that the framers meant exactly the opposite of "English law."
The framers of the Constitution were, of course, well-versed in the British common law, having learned its essential principles from William Blackstones Commentaries on the Laws of England. As such, they knew that the very concept of citizenship was unknown in British common law. Blackstone speaks only of birthright subjectship or birthright allegiance, never using the terms citizen or citizenship. The idea of birthright subjectship is derived from feudal law. It is the relation of master and servant; all who are born within the protection of the king owe perpetual allegiance as a debt of gratitude.
According to Blackstone, this debt is intrinsic and cannot be forefeited, cancelled, or altered. Birthright subjectship under the common law is thus the doctrine of perpetual allegiance. Americas Founders rejected this doctrine.
The Declaration of Independence, after all, solemnly proclaims that the good People of these Colonies. . . are Absolved from all Allegiance to the British Crown, and that all political connection between them and the State of Great Britain, is and ought to be totally dissolved. According to Blackstone, the common law regards such an act as high treason. So the common lawthe feudal doctrine of perpetual allegiancecould not possibly serve as the ground of American (i.e., republican) citizenship.
You're right---you ARE wrong. The law in effect when Obama was born was that in order for an infant not born on US soil to be a natural born citizen, ONE parent had to be an American citizen OVER AGE 18 (Obama's mother was not yet 18) if the other was a foreign national. This has since been changed (precisely how, I don't know), but is what would apply to the existence or non-existence of Obama's "natural born citizen" status.