"Could be because, there is no legal defintion of "Natural Born Citizen"."
For the sake of argument lets say that's true. Then my point is valid. I was disputing a birther claim that two citizen parents are in fact required. That there is no such legal definition falsifies that claim.
And none that supports it either. There is however considerable historical evidence supporting the "child of citizens" notion, and even some legal support, in Perkins vs. Elg 307, 307 U.S. 325 Ms. Elg was declared, by the DC circuit, to be a "natural born" citizen. This was supported by the Supreme Court, even though only her citizenship was at issue. Both her parents were naturalized citizens *at the time of her birth*, which was in the US. Of course that doesn't prove that if she'd only had one citizen parent, that they wouldn't have done the same. (I've not been able to find the DC Circuit's ruling, and am going on the citation to it in the Supreme Court's ruling, they may have expounded further on the matter)
One major historical support for the "child of citizens" requirement for natural born status comes from one of the authors of the 14th amendment.
I find no fault with the introductory clause, which is simply declaratory of what is written in the Constitution, that every human being born within the jurisdiction of the United States of parents not owing allegiance to any foreign sovereignty is, in the language of your Constitution itself, a natural born citizen; but, sir, I may be allowed to say further, that I deny that the Congress of the United States ever had the power or color of power to say that any man born within the jurisdiction of the United States, and not owing a foreign allegiance, is not and shall not be a citizen of the United States.”
John A. Bingham, (R-Ohio) US Congressman, Architect of Section 1 of the 14th Amendment, March 9, 1866.
This statement is documented in Cong. Globe, 39th, 1st Sess., 1291 (1866), Sec. 1992 of U.S. Revised Statutes (1866), Cf. U.S. Const. XIVth Amend.
Notice that the first part says "of parents not owing allegience", while the second says "any man born within the jurisdiction of the United States, and not owing a foreign allegience". So in the first case it is the parents that must not have foreign alliegience, in the second the "man" himself. The first part describes "natural born" citizens, as used in the Constitution, the second "citizens".