That’s what I’ve heard as well, but I guess I don’t get it.
How is you (a human) fapping to ponies any different from you fapping to humans AND ponies in the same image?
I like human-on-furry (though I haven't come across it in a while), but not a lot of human-on-pony. That's because there's often a physical-size difference that makes it seem as if the pony is the human's pet. That sort of relationship doesn't mix with sex.
(That Palutena x Celestia drawing, though, is great.)