I don't smack, and I think there are more effective forms of discipline, BUT...
I find the argument 'It would be wrong if your husband/ a stronger person did it to you, so you shouldn't do it to a child' to be a redundant one.
When raising a child there are many things that are appropriate for a parent to do, that would NOT be appropriate for one adult to do to another. Time out, grounding, telling off, removal of possessions, restraint (e.g., child seat/ highchair/ cot), deprivation of liberty, etc.
There are other arguments that I think are more valid when discussing whether or not smacking is acceptable. To say 'it's wrong because you wouldn't do it to another adult' doesn't really work when there are other things that fit the same description and are thought of as acceptable.