I have always found the appeal to only come true in 2D art, not in 3D pictures. As I see it, a man often watches porn to get off, and fantasizes about the situation. This causes the male porn character to be the viewer-insert character, the one whom the viewer juxtaposes themselves over.
Now, with futa (in the small/average penis variation), it is often normal sex being had, one girl and one futa. In this situation, it is still possible to have the futa as the viewer-insert character, yet having all of the perks of two cute females on screen instead of just one.
It's my hobby to rationalize odd fetishes. Whether or not I care for it, I can usually speak for a fetish for quite a while. As such, do not take any of these words as my personal feeling, only as how I can see others feeling.