It seems to me completely obvious to any rational person that children do not HAVE to be smacked. No teachers, child-minders or nursery employees smack children, & still manage to keep them safe & secure & to manage behaviour by other means.
Numerous parents bring up their children without smacking & manage to teach good behaviour through the use of consistent boundaries, positive reinforcement & other methods that are at least as effective as smacking, & without the negative side-effect of suggesting that hitting people smaller than oneself is sometimes acceptable. I'm one of them, as is dh: & as we've both worked as teachers, we've also managed behaviour without resorting to physical force in that context.
I still think it's unlikely that an occasional smack is going to do any lasting damage to a child, within an otherwise healthy parent-child relationship. My mum smacked me on a few occasions - just a single slap - & while I don't remember it achieving anything positive at all, other than to upset me, I don't think of it as a terrible form of abuse either. My mum was a human being, she didn't always behave perfectly. If I'd ever smacked my kids, I'd have apologized, but I don't think I'd have felt eternally guilty. I have shouted, & apologized then too.
What I have no sympathy with is the view expressed by these other mums, OP, that smacking has to happen, that it's inevitable, even good for the child. I find that disturbing, & agree with the person upthread who described this as lazy parenting. It IS lazy, because there are plenty of other ways to deal with difficult behaviour, so why would any parent deliberately & repeatedly choose to smack their child rather than make the effort to develop more effective ways to discipline?
When I was teaching I came across very few children whose behaviour couldn't be managed if enough thought went into the best ways to do that. Any child whose behaviour is so terrible that they don't respond to non-violent discipline is sure as hell not going to be helped by being hit.