After 3 minutes philosophizing on the subject I believe I've developed a groundbreaking postulation which aims to lay the groundwork for all possible answers to the futa question.
It places each pornographic content viewer somewhere on what I've dubbed the Attention-Perspective-Pornographic-Continuum:
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On the X-axis is perspective. Do you assume the role of a pure
witness to the action (voyeur) or do you use your imagination to assume the role of a participant (self-insertion)?
On the Y-axis is attention. What are you looking at? The person being fucked (fuckee) or the one doing the fucking (fucker)?
In typical, heterosexual porn, I hypothesize that our attention is predominantly on the female (fuckee) and that most (?) viewers assume the perspective/role of the fucker (self-insertion).
The point of futa/shemale porn seems to emphasize the futa/shemale (the fucker?) as the point of interest, although it isn't necessarily clear whether the viewer assumes the role of a 3rd party witness to the fucking or imagines themselves as part of it. As I don't watch futa or shemale porn, I can only guess that such viewers would find themselves on the lower-left quadrant. That is, attention on the fucker, assuming the role of voyeur.
Thank you for coming to my TED talk.