In fact, when "imagining doing the fucking" you're arguably doing a heavier abstraction, since a fictional character *only* exists insofar as you conceive of them; while fantasising about another by necessity strips them of layers of hard reality — their pasts, their emotions, hell, their biology. In anything, it's even more disingenuous.
And springing off that point: people see people in nonhuman things all the time. The internet is full of pictures of lewd trees, for instance. Ultimately, what we're attracted to are themselves abstractions; the undercurrent of machinery to how our brain works, lines, curves, sensations, just like allow us to see an electrical outlet as a smiley face. So, in turn, a drawn character might be EASIER to fetishise than a real person, since they're only the exact necessary requirements of arousal; concentrated neural patterning, if you will.
So really, I must turn your question back around on YOU, since I don't understand how someone wouldn't find this automatically intuitive, or be so trapped in the pedantic necessities of the real world not to be able to imagine it.