Obama isn’t the first President with paternity doubts.
Bill Clinton (William Blythe) was born only seven months after his “father” returned to the US. His “father” died a couple of months before Bill was born a full term baby with only seven months gestation.
He was never adopted although his name was changed to Clinton so his younger brother would have the same surname in school.
Who was Bills real Daddy? Was he American? Who knows?
We have pretty much exactly the same information about who is “really” the father for every other president. Same as for every other person, in fact, on whom a paternity test has not been done.
Statistically, there is an excellent chance one or more of our 40+ presidents has not been the biological son of his mother’s husband.
Luckily, the law does not care who the “real father” of the child of a married woman is. Her husband is by definition the legal father and it is almost impossible to get the law to recognize otherwise, even with indisputable DNA evidence.
IOW, obsessing about who Obama’s “real father” may be is an exercise in futility. The law just doesn’t care. It’s irrelevant.