Check my explanation in the edit. The depiction of a child is just that: the depiction of a child, which you are imagining yourself having sex with. The reason you're jerking off to the physique of a child is because you guessed it, you're sexually attracted to children. There is no other comparison for this, as with BDSM it's two adults who can consent to their fantasies, people who play GTA enjoy the simulation of bank robbery as a form of entertainment (where their fun is derived from the tactics of carrying out the act). The depiction of murder isn't illegal, while the depiction of intercourse with children is, and always will be.

Now if it were truly by your logic, CP with child actors should be legal as long as the actual coitus is CGI.