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The odds are that the FOX people didn't intend to suggest that Obama and Michelle weren't married. They meant to slander them, of course, but I'd bet that they had no clue of the specific meaning of the term "baby mama".
No, they were just displaying the usual complete cluelessness about black culture that's typical of all the right-wing whites on FOX - though I'm sure none of them have the slightest inkling of the depth of their ignorance.