I'm another who hates smacking but dh had fewer qualms about it when dd was a toddler. I wasn't smacked much but got the occasional one on the thigh. The most terrifying experience re parental punishment I ever got was when my mum lost it (I had been dreadful), sent me to my room and came up later to smack my bum. I did deserve it, but actually she hadn't really the heart to carry it out, and so though I 'got away with it' I never did that particular thing again.
I will however let you all in on a horrible 'secret' of which I am heartily ashamed:
dh, now 7, in the last 6m developed a habit of rushing up behind either me or dh and just careering into whichever one of us, breaking her rush with her hands wamming into the small of our backs. Now we both have rather chronic physical problems and believe me this was completely unacceptable. After a couple of months of this, where we started with the 'don't do that' and wound up with 'DON'T DO THAT', she'd seen the results (from causing pain to rendering us unable to stand up) none of this worked. Naughty step, removal of toys/privileges etc - all to no avail. A month ago she did it to me and Yes, I turned round and smacked her on the bum (pathetic smack as my arm got caught up in things I was carrying, so I barely touched her) but she has never done it since.
Personally, despite the above, I don't like smacking, I think 7 is far too old to be smacked, I am horribly ashamed of myself, but every so often I remember that incident and I realise I haven't a clue about anything really!