Punishment is never particularly pleasant, of course. But is there any principled reason why corporal punishment must be worse than other kinds? In practice, it may be more subject to escalation and abuse. But suppose we could avoid that risk (e.g. through mechanized administration). Would that make it okay, or is the infliction of physical pain different in principle from other kinds of punishment?
If anything, momentary pain seems like the perfect punishment. We are strongly motivated to avoid it, and yet -- unlike incarceration -- it is over in moments and has no lasting ill-consequences. (Prisons should be used only for purposes of removing threats from society. They're too inefficient for mere punishment.) Perhaps fines and "community service" are better still, since they produce benefits to others rather than merely imposing harms. But corporal punishment could replace jail-time as the ultimate punishment, for when all others have failed. Why not?
Criminals aside, consider children. Some people claim that smacking is an inexcusable act of violence, intrinsically "abusive" no matter how light it may be. But again, why is physical discipline picked out for special treatment? Isn't this arbitrary?
Perhaps we have (indirect utilitarian) reason to promote the norm that one's body is inviolate. But parenting and legal punishment are a special cases, where we may allow things that we wouldn't normally allow (e.g. locking people up against their will!). So I don't see why we couldn't do just as well with a more restricted norm of bodily autonomy that can be overriden by appropriate authorities (i.e. a young child's parents, and the legal justice system). It needn't have broader implications for how we treat each other in society.
Compare the extreme case of torture. Torture is intrinsically problematic because it essentially involves the use of extreme pain to induce psychological breakdown (and subsequent compliance). The mark of abuse is that it leaves the person physically or mentally "broken", unable to function properly as a fully autonomous agent. This consequence is atrocious.
But if light physical punishment can safely avoid such effects, what else is left that's so objectionable? Perhaps being physically dominated by another induces feelings of helplessness. But it is the domination -- i.e. arbitrary power -- that's the problem here, rather than the infliction of physical pain per se. I agree that this is a severe risk in practice, but suppose for sake of argument that corporal punishment could be delivered in a measured and non-dominating way. Would it still be objectionable, even then?