Practically? What does that even mean? Of course there are moral questions regarding the practice of cannibalism! Cannibalism still existed as a traditional practice in the twentieth century, so it is a moral question. And ethically speaking, there are many things to say about it.
The reason I brought it up is because I think it sets a line when discussing the right to kill for food: there are cases in which no harm comes of it, and other cases in which the practice would be highly objectionable.
I am making the case that in order for you to claim a moral high ground against an actor, the actor needs to benefit from the action they do. In this case, they don't. If you start a human carcass diet, you poison yourself. Your body reacts to it in terrible ways. Clear enough?
It seems to me that you are confusing utility with morality. The fact that eating human flesh might not be good for us (which, being a questionable claim, let's take for granted for the sake of this discussion) is beside the point. The question is not "is it good for us in a dietary perspective?" but "even if it were good, would it be acceptable?".
A consequence of what you are saying is that it is impossible to claim a higher ground against a murderer if the murderer doesn't benefit from the murder or if an accuser can't make a case for it.
Besides, benefits are relative most of the time: you say there isn't one, but someone else may just as well argue there is.
But if you absolutely need a benefit, so be it: imagine someone eating human flesh simply to satiate one's appetite for it. It would still be a morally questionable practice because of other values put into question.
Anyway, this was a thread about fish feeling pain, so I insist: maybe there are many species we should treat in a different way, independently of whether they feel pain or not. Some scientists have reclaimed the statute of non-human personhood for great apes and cetaceans, for example. So killing them painlessly still raises questions.
The reason I brought it up is because I think it sets a line when discussing the right to kill for food: there are cases in which no harm comes of it, and other cases in which the practice would be highly objectionable.