“And they get all that from the opinion of a guy that wrote a book that some Founders might have read that is not referenced in any founding document.”
The Naturalization Act of 1790, a founding document, directly defines what a natural born citizen is. It defined it as a person born of citizen parents. And, citizenship descended through the father only, according to that Act.
That Act was written by the ones who wrote the Constitution, using the accepted definitions of their day, and that concept was placed into the Constitution.
Actually, not a founding document. A law made after the founding.
And the children of citizens of the United States that may be born beyond Sea, or out of the limits of the United States, shall be considered as natural born Citizens: Provided, that the right of citizenship shall not descend to persons whose fathers have never been resident in the United States: Provided also, that no person heretofore proscribed by any States, shall be admitted a citizen as aforesaid, except by an Act of the Legislature of the State in which such person was proscribed.
Don't confuse the plural "citizens" to mean both parents, like the birthers claim. It is simply a multiple to collectively include the citizen parent (father) of any citizen child and all citizen children. There never was a two-citizen-parent requirement. Once women were no longer second class citizens, the requirement that citizenship pass through the father became obsolete. It didn't fully go away until after Obama's birth, however, thus the importance of where he actually was born.
Clearly the Founders considered that some citizens would spend significant time out of the country - we were, after all, a maritime nation and a frontier nation at the same time. They also considered that children would be born to those citizens and themselves spend substantial time out of the country. Thus the requirement of only 14 years of residency, with no mention of place of birth, only status at birth, to wit: citizen at birth - the specifics of which are delegated to Congress in the Constitution, which is a founding document. Fourteen years represents only 40% of the minimum age requirement. And that assumes someone could manage to get elected at 35. For someone the age of Donald Trump that requirement is only 20% of their age.
Birthers make a lot of claims based on wishes and hopes and not much else. A little logic goes a long way to sorting out the truth. Citizen at birth, 35 years old and 14 years a resident. That's it.