A smack was the normal punishment for me as a child. Back then it was acceptable, all my friends were smacked.
It's when a parent leaves you with bruises, actually hurts you (rather than a smack which stings a little and shocks you) that it's bad.

I've a volatile temper, so I wouldn't smack my (hypothetical) kids for fear of losing it. I'd need a room to shut myself in and scream when they drove me nuts and maybe when I came out they'd have stopped.