That's pretty easy - because prior to the Declaration of Independence, no one born before that day could be "Natural Born" citizens as they had been born in British sovereign territory.
This issue has been settled law for quite some time. Since the Constitution did not completely spell it out, it is likely the meaning of the phrase was understood as being based on the English Common Law. This was ruled upon by the U.S. Supreme Court in 1898 in United States v. Wong Kim Ark, where it held that It thus clearly appears that by the law of England for the last three centuries, beginning before the settlement of this country, and continuing to the present day, aliens, while residing in the dominions possessed by the crown of England, were within the allegiance, the obedience, the faith or loyalty, the protection, the power, and the jurisdiction of the English sovereign; and therefore every child born in England of alien parents was a natural-born subject, unless the child of an ambassador or other diplomatic agent of a foreign state, or of an alien enemy in hostile occupation of the place where the child was born. III. The same rule was in force in all the English colonies upon this continent down to the time of the Declaration of Independence, and in the United States afterwards, and continued to prevail under the constitution as originally established.
That I understand and was spelled out in the requirements for President. The Founders referred to those that were residents of the Newly Formed Country that were subjects of Britain. They had to be grandfathered in, otherwise, no Presidents.
In rulings and Amendments added later, there in lies the confusion.
As to loyalty, I have NO DOUBT that Jindal, Rubio, Romney (barf) have no other loyalty to any other country than to which they were born in.
I have NO DOUBT that they would be many times over, a better President than the current one.