I always have fun posting this:
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Maybe Stanley got one of those little black 'honeys' pregnant at that bar where Grandpa and Mr. Davis used to hang (literally) out. Then once the baby was born, they had Stanley 'claim' it was hers (and the grandparents put a birth announcement in the paper to make it look good).
Consider that Stanley didn't hang around too much and left the grandparents to 'raise' Barack. The marriage to Obama Sr. seems to have been only to have been a way to make it look good, and hide any question of who the father was.