An "orally-fixated" person is one whose pleasure comes from the mouth, from putting things in and out of the mouth (a singer, a gastronome, a smoker, even a bulimic). In terms of Freudianism, all of these activities, for an orally-fixated person, are erotic, if not explicitly "sexual." So it could just be that she enjoys singing. But Shakira is giving a BIG wink to the idea that she gets all of her pleasure from, well, putting things in and out of her mouth. We all know what she means - penises. She's claiming that she gets off on performing fellatio.
Is that true about her? No idea. Is it a great marketing ploy? Seems to have worked pretty well.