It isn't comparable to the domestic situation between a man and woman because a man hitting a woman is not responsible for teaching that woman everything she ever knows or keeping her safe from herself. The balance of power is different and violence in this relationship is likely to be of a much higher degree, be symptomatic of very severe issues and has a very different meaning.
No, it isn't comparable now, because a woman can leave. She is not totally powerless. She has legal recourse and society on her side. A child does not. I'm not sure how you can argue that absolute power and total defencelessness makes violence less problematic, frankly. In any normal assessment, that's an aggravating factor.
A child doesn't even know that it's wrong to be violently assaulted at the whim of the person with absolute control over every aspect of their lives, and reading this thread, it seems quite a lot of adults don't know that, either. And as I mentioned earlier, societally it used to be exactly the same. Men beat their wives to discipline them, and that was seen as completely reasonable because wives were regarded as essentially inferior, and owing their husbands obedience in much the same way. All that's changed is societal sanction.
I'm also baffled by the notion that it's somehow a justification of hitting a child that a man hitting a woman is not responsible for teaching that woman everything she ever knows or keeping her safe from herself. You think it's a good idea to teach a child that violence solves problems, is acceptable, and the only option when someone really pisses you off? Kids learn by example, so all it teaches is that you can force someone weaker to obey you without question, and hurt them if they refuse. I don't actually like the idea that my child will be in a playground with kids being taught that. It does not seem to me to be a healthy or sane lesson to be teaching.
Parenting is hard. And I have a lot of sympathy for adults who lose it and smack very occasionally, because they are surrounded by a culture that says it's okay, and nobody can wind you up quite like your own family can. What I have no sympathy for is people who then try to rationalise their own violence towards their kids as being somehow a sensible parenting strategy, and not actually violent at all, and if only all of us beat our kids like them the world would be a better place. Ask any social worker: most problem kids do not come from homes which abhor violence. Quite the reverse. And it's plain ridiculous to claim otherwise.
Calm authority works. It's exhausting, sure, because gently correcting behaviour and talking through why things are unkind/silly/unacceptable is time and energy consuming, and often you have to do it when screaming on the inside. But it's a parent's job. I don't have the right to hit my kid, because I don't have the right to hit anyone. Violence is wrong.
What really interests me about this is that when you call smacking what it is - violence aimed at getting a child to do as an adult wants, without any attempt to teach them to regulate themselves via thought/conscience - people get antsy. They insist that they aren't talking about assault, violence or beating; oh no, just smacking. It's a real example of Orwellian doublethink. If you really want the right to violently assault your own child, then at least have the honesty and guts to state that that is what you want. Mealymouthed euphemisms don't alter the bare facts: it's using violence and the threat of violence to force your will on a child. And I doubt many of the people briskly saying there's nothing wrong with a smack would be happy to do that. If you can't speak honestly and truthfully about what you're actually doing, then maybe you shouldn't be doing it.