Actually it's not. There's a legal principle of "presumption of legitimacy" that goes back to the Middle Ages and beyond. The child of a married woman is legally presumed to be legitimate.
“the unsubstantiated claim that he was born in Hawaii”
There are POP (Presumption of Paternity) Laws in most states that maintain that ANY child of a marriage is the Husband's legal child. Having said that...we don't know that Obama Senior and Stanley Ann were ever married. I have been married and divorced twice and in NEITHER divorce were I or my ex's asked to produce evidence of a marriage license, marriage certificate...anything at all. And still divorces were granted. I seriously doubt they were married and since Senior WAS ALREADY married, his and Stanley Ann's marriage would have been null and void anyway.