Whether in shotacon images, written stories, any other kind of fiction, or (tragically) in real-life itself, anytime ano-rectal penetration takes place between a child or adolescent and an adult male (or between a younger child/adolescent and an older, more developed one), it is almost invariably the shota-- the younger, less-developed one, who is on the receiving end of it. It is thus the tender, young lad who is placed at far-greater risk of infection and injury and who alone bears the burden of the pain and other trauma and complications involved.
Much the same holds true for fellatio as well, where, again, it is overwhelmingly the innocent young lad who must endure the invasion of mancock into his tender mouth and the foul blasts of putrid manslime that follow. (the latter often landing on the poor tyke's face as well, and even reaching-- and stinging-- his eyes)
Having said all that, I hasten to also point-out that shotacon art is by no means limited to depictions of the brutality, predation and sadism that I have just described. One need look no further than this very board to find many images that are free of all of these. Images that portray boys in all their natural glory. Boys at full-mast, frolicking freely and safely. Many of the more wholesome shotacon images are even heart-warming and sentimental, capturing a special innocence and sweetness of boyhood.
Unfortunately, everything-- from the most innocent and wholesome to the most vile and horrific-- all of it gets lumped-together, without distinction, under the broad category of "shotacon". I know I am not alone in my frustration at this. There are no doubt others who share the views and feelings that I have expressed in this and other posts-- even if we are in a distinct minority among our fellow shotacons.