I love to point out how, very recently, it was culturally accepted and expected for men to keep their wives in line with violence and slave owners/gov to keep slaves in line with violence.
"We now recognize the obvious and gross immorality of those, the evil of it. Violence against kids is next to be seen for the brutality and wrong that it is; a change which is already well underway with corporal punishment declining in homes and schools nationwide. You don't want to be one of the last people doing, or even defending, something like that do you?"
I love the framing of someone defending spanking (or the state) as akin to those that defended slavery or wife-beating, right to their face. Like Stef points out, our side are not the ones that should be on the defensive!
Of course this is just Stef's concepts put in my own voice.