A big problem with 3-D is that it's better at separatinng foreground from background than it is actually giving you a feeling of depth. It's like putting a cardboard cutout of a person in front of the Eiffel Tower. You get a sense that the cutout and the Tower are physically separated, but the person looks flat. 3D usually works best when you have a foreground, a middleground and a background that are significantly different distances.
But putting stuff right up in your face usually doesn't work. Your eyes have to converge and often it's just too much, your brain can't put the images together and the result is a disaster. So instead of a closeup of a woman's spread legs and a cock pumping her pussy, it looks like a double-exposed mess.
There may be good 3D porn, but I haven't seen it. (If someone wants to point to a good one, I'd be happy to check it out.)
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