The very sexual ones freak me out because they falsely imply an actor's willingness to participate in a particular type of scene. Also? The bodies they typically choose for those things look nothing like the actor's so it's double weird, like seeing an eagle's head on a lion's body.

I've seen some manips that aren't sexual and they fail to freak me out but I don't really "get" it, either. I suppose it's good practice on Photoshop and it obviously gives people enjoyment. Luckily for them, their fun doesn't depend on my getting it. :)