I think this discussion is really about the way we view strong human emotions, and the idea of "destructive normality" e.g. that the "normal" human experience is free of suffering, people are in control and act reasonably at all times, their actions are commensurate with their beliefs etc.
It saddens me when you read people who describe their mothers/fathers/carers as "generally awesome" and then say that the few times they lost control were unacceptable. So, you got hit by someone who had lost control for a second. Someone who hugged you every time you cried, who made sure you ate and slept regularly, who lay at night awake worrying about you, who shared their thoughts, interests and experiences with you, who read to you, who told you they loved you, who took an interest in your friends and schooling, who considered your education and spiritual welfare as serious matters, who taught you how to wash and dress yourself and tidy up your environment, who laughed with you and worried when you were sad, who generally gave you a childhood of routine and predictability... but...had very rare moments in which they behaved in ways they were not proud of?
So you hated them at that moment. So you can remember these moments with bitterness.
My question here is... so what? Really, so what? If the experience of being smacked taught you that an adult who was generally loving, predictable and kind could lose control on occasion and experience fear/anger/rage etc, what's the issue? Do people really believe that any parent can raise a healthy child without sometimes losing control? Perhaps they won't hit, but who will parent without saying something or acting in a way that is not in line with their values at some point? Or alternatively, who believes you can parent the way you believe is best and not have that style inadvertently impact upon your children in negative ways?
It's called humanity, people. I think in reality very, very few people in this culture at this point in time either want to smack or believe it is okay. Some of them may sometimes lose control. I did once. My son (3) randomly turned around and pushed his baby brother's highchair over with force, hitting his brother (9 months) with force in the head. I reacted before I even realised. I was literally going to pick up the baby with one hand and I slapped him on the bum with the other hand. I didn't wallop him, but was it a considered decision that I can defend? Erm, no. Did I believe it would be teaching him a lesson... or that it was a good idea? No, again. I got a tremendous fright and I also felt angry. It's not supportable but it was human. I apologised and we cried together for a few minutes, all of us, and then we got on with the day.
The important thing is that you return to your values, time and time again, not that occasionally you may find yourself deviating from them.