There are several motion capture techniques... The have body suits, lazer beam rooms, animatronic puppets w/ motors that feedback movements to the animation program...
Well we already have really fucked up anime that you can buy -- monsters raping women with thousands of penises ripping their bodies to pieces -- it's all over the internet...
I'm sure we will see more motion captured porn. Some of the reasons it hasn't been done yet is the rendering times for realistic human bodies... also porn is very much about skin contact, subtle things like sucking, jiggling, etc... ;) (ie: object collisions) Very difficult to animate and the way skin moves and deforms for example when you squeeze a breast -- it would require very complicated motion capture and most of it would need to be edited by hand by artists -- very tedious and expensive... If you just did straight motion capture of two figures fucking it would look silly.
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...from the nectar of the Bone flows all that clicks...