I am sure we have had presidents who were born to unwed parents. But if you mean something different, then it makes sense.
If Obama Sr. was indeed married to his wife Kezia in Kenya in January 1957, following which their son Roy was born in 1958, then Obama Sr.'s marriage to Ann was bigamous and void under Hawaii law. That would make Obama Jr. illegitimate.
Obviously, these circumstances were entirely outside the control of Obama Jr.
I am not sure there is any other president as to which there has even been an issue of illegitimacy, including Bill Clinton. With the exception of Obama, historians and biographers have gone exhaustively through the background of all of our presidents. Is there some president in particular you were thinking of?
You make a good point. We use the word illegitimate assuming the context is clear. Who knows if Obama's father was legitimately married to Stanley Ann. We must trust Obama's words, and trust that "Dreams from my Father" expressed who he is. There should be no legal loopholes in the meaning of our framers, who clearly intended that our president have parents with allegiance for our nation and Constitution - criteria Obama and his father clearly fail to satisfy. Let me try, without burying you in quotations, to explain the meaning of our framers. Then I'll point you to a thread created by rxsid which has more references than you'll probably have time to read.
When we were a colony the group who chafed at rule by a remote oligarchy, our founders, was a minority. After we declared independence there was great hardship. Some claim Royalists were seventy percent of the public. How then do you chose someone to be your king, temporarily, and to command your armies whose allegiance you can trust?.
The core idea, which appears in quotations of Aristotle and Cicero, is that allegiance can be vouchsafed first by knowing the allegiances of a person's parents, and second by assuming he or she will have allegiance for the soil upon which he was born. The English actually adopted those very rules in 1608, even making the children of "natural born subjects," wherever they were born, natural born subjects. The King wanted people to conscript and tax. The new nation wanted citizens who wanted individual freedom - very different goals, and so a natural born subject is quite different from a natural born citizen. Those ideas together comprise the condition of being a natural born citizen.
That condition has other names. Framer, Dr. David Ramsay, used the term "birthright citizen." Vattel's first edition was written in the diplomatic language, French, and Vattel used the terms "Native" or "indigene". Marshall cited Vattel, as did Chief Justice John Jay. Chief Justice Morrison repeated that definition, as did Chief Justice Charles Evans Huges in 1939 - "born on the soil of citizen parents." .
There is a long history of legal philosophy called loosely the law of nature, and later, law of nations, composed of wisdom dating at least back to Aristotle which gained adherents such as Leibniz, Pufendorf, Grotius, Wolff, and Vattel - mid seventeenth through the nineteenth century..
Swiss philosopher/jurist, Emerich Vattel, who happened to write particularly clearly wrote the book upon which our framers most depended when they needed, with great urgency, to establish a constitution. The English Constitution, as Hamilton observed, was nonexistent, and English common law was hundreds of volumes of often inconsistent, sometimes self referential, though elegantly phrased, assertions which wouldn't damage the barrister's standing in the royal court. Everything was about pleasing the King or Queen. Our framers needed rules to protect free men from the powers implicit in a central government. To learn about the distain our framers had for English Common law, read framer and justice James Wilson's Commentaries, or browse through Hamilton's letters.
The book, Law of Nations, was the text chosen by Thomas Jefferson as the text for the first law school in the U.S. created at the College of William and Mary in 1779. Before that colleges were controlled by the crown. Law schools were not necessary since we were all British subjects. (It may be that they were not permitted - someone may provide those data).
The Law of Nations (which you can find on line or in an elegant edition published by Liberty Fund on Amazon) was the first book checked out of the New York Public Library by George Washington, who arrived at his New York office in 1789 without his library, and was the only book on his desk, according to reporters waiting to interview the new president. It was the most cited judicial reference for the thirty two years between 1779 and 1821. Chief Justice John Marshall, who attended William and Mary in 1880, cited Vattel when he clarified the importance and types of citizenship for a case called The Venus in 1814. The only citation I'll suggest, because it lead me to trust the truth of this remarkable farce, is The Venus, 12 U.S. 253. at paragraph 289. If you want to learn more, rsxid has provide a rich thread with hundreds of references right here on Free Republic http://www.freerepublic.com/focus/f-backroom/2512143/posts
So the legitimacy in this case is Constitutional legitimacy. The words about what it means are available, and were repeated in house and senate hearings often between 1998 and 2008. The eligibility criteria were to help assure the allegiance of our chief executive. Many think we should amend the provision to include children of military, and I agree.
Orrin Hatch, initiated an attempt in 2003 to allow Schwarzenegger to run, born neither on our soil or of citizen parents; his amendment had very little support. The Constitution is our law unless we change it. Obama has ignored it, and, so far, been challenged by few representatives. One who did, Congressman Nathan Deal of Georgia, was promptly responded to with ethics charges. He resigned to run for Governor. During mob rule, that was clearly the wise thing to do, and probably why there aren't other voices.