@Goomy pls
"But it doesn't promote good behaviour, just punish bad behaviour. And like timeouts, the child will just look for ways to do bad things without being punished."
This is true, and thank you for noting that this is also the case with time-outs, so this isn't an argument against spanking specifically. This is just the nature of punishments with children. As children grow, so too do their intellectual and reasoning capabilities and the parent is able to rely more on explaining why certain behaviours are wrong and others are right. That's why you don't spank a 13-year-old for when he/she misbehaves. The parent whose only tool for shaping their child's behaviour is spanking and punishment is a shitty parent, because they don't nurture the development of their child into a moral person. But to promote good behaviour, you also have to discourage bad behaviour, and when the child is too young and immature to comprehend anything beyond the immediate consequences of their actions, spanking is just one means of doing this. It's nothing special. You're not suggesting we do away with punishing children entirely, are you>?
"Not to mention that in many cases the parent is larger than the child and could, if unintentionally, seriously injure the child."
Given that 1) the gluteus maximus is the largest muscle in the human body, therefore more cushion (I feel kind of stupid now saying the same thing as Davey here, but facts concerning human biology don't change just because a retarded fundie is aware of them) and 2) that you spank with your open palm (increased contact area, Pressure=Force/Area, therefore reduced overall pressure) you'd have to hit really hard to seriously injure your kid by slapping them on the bum. Also, if you're so unfamiliar with the limits of your own body that you honestly don't know how much force to apply without hurting your kid, then yeah, you shouldn't be spanking your kid.
"An absolutist view would be to say that spanking always produces violent criminals. Obviously it does not. But it can, and it can cause psychological harm, so all spanking should be at least questioned, as with any other punishment or any incentive."
Yes that would also be an absolutist view, but that's not what I'm calling you out for. Typing "Spanking is abuse!", bolded, underlined, italicized, all-caps, large font, is just as much of an absolute statement since it implies that there's no room for exceptions. You're right, it can cause psychological harm, but so can any punishment if taken to extreme lengths; spanking isn't special. That's why you need to look at it as a matter of degree. So I'm glad you included "as with any other punishment" in there as well.
"As for slapping your kid on the bum, it is an act somewhat sexual in nature"
How? Maybe if you live in an oversexualized society that frequently implies a sexual side to otherwise innocuous acts. You mentioned in an earlier post that the buttocks is an erogenous zone, so therefore spanking on the bum is sexual. Wikipedia, which I hope we can all agree is a pretty neutral and unbiased source of information, defines an erogenous zone as "an area of the human body that has heightened sensitivity, the stimulation of which may generate a sexual response, such as relaxation, the production of sexual fantasies, sexual arousal and orgasm." [Emphasis added] Given this definition, the idea that a child's bum is an erogenous zone seems absurd to me. An adult's, sure, but not a child; children are obviously not sexual beings. The only other way it could be considered sexual is if the parent derives sexual pleasure from spanking their child, in which case the problem lies with the parent, not the act itself.
"and could potentially screw up nerve endings or blood vessels in the area."
Maybe if you're an amphibian and you breathe through your skin. In humans, the outer, keratinized layer of skin, the epidermis, has no blood vessels and no nerves. You'd have to hit really hard to significantly damage blood vessels in the underlying dermis (which heal anyway) and if you are, you've crossed the line from harmless discipline to physical abuse causing injury, and you're a shitty parent. You'd have to hit even harder still to damage any nerves, and even if this does happen (because you're a shitty, abusive parent), they'll likely just regenerate anyway. Notice how you don't lose feeling in your arms and legs when you fall while learning to ride a bike? Really, if the human body were as frail and delicate as you make it out to be, it'd be a wonder that natural selection hadn't weeded us out as an inferior species eons ago.