It is not true that in order to be a natural born US citizen that both parents must be US citizens. Only one must be, but there is both an age and a residency requirement for that one if the baby is born out of the USA.
I think I have that right.
For example, Obama would not be a natural born US citizen, even if his mama were a US citizen, if he had been born out of the US prior to her having attained X age + X years of residency over a certain age.
I was basing these stipulations on the following page from theobamafile.com
http://www.theobamafile.com/NaturalBornChart.htm
Note in the section defining natural born citizen there is a judicial citation supporting the two-parent rule...
NATURAL BORN CITIZEN
Both are U. S. Citizens
AND
Born in the U.S. mainland
Per the following...
US Constitution Art. II, Sec. 1, Cl. 5. U.S. v. Wong Kim Ark, 169 U.S. 649 (1898)
Perkins v. Elg, 307 U.S. 325 (1939)