Weren't men at one time allowed to beat their wives

There is no connection between the beating of wives or for that matter any other adult and the smacking of a child.

A parent is not responsible for the behaviour of other adults, they don't have to protect them from their own actions neither is it their job to teach them how to behave whereas a smack then and there may be the most appropriate punishment for a child in a particular situation.

A child waiting with their mother at a bus stop, tries to dash across the road and receives a smack on a leg which is brief and only remembered as being connected with the action, or they are told they will be sent to bed when they get home for being naughty and so is upset for the rest of the journey, resentment probably meaning they forget why.