You believe what you want to believe.
It’s strange that so many people say that they were taught this but no one has been able to come forward with a single textbook from primary school through college texts which states that two American citizen parents are required in order to qualify as a natural born citizen.
And of course in the eras prior to DNA evidence, there was no way to determine with absolute certainty who a person’s father really was. Husbands are not always fathers.
Even today there is speculation about who Obama’s “real” father might be and the two most mentioned names would both make him a natural born citizen (Frank Marshall Davis & Malcolm X, born in Kansas and Nebraska respectively).
And until birthers can find a single textbook citation that defines NBC as requiring two citizen parents, I'll continue to believe that those claiming they were taught this in school are full of crap.