"While the buttocks is not a sexual organ, it is a sexual part of the body, similar to women’s breasts. Children are not sexually mature, but that does not mean they are incapable of feeling sexual sensations. Spanking can greatly interfere with a child’s sexual development, modesty, and their understanding of bodily autonomy. Spanking an adult would be considered a sexual offense and it should be the same for children.".
This is just an opinion piece. I read a much better piece about it before, but I'm having trouble tracking it down.
I don't necessarily agree that spanking is always sexual abuse, but there are accepted practices in modern evangelical Christianity centered around spanking that very much are.
The bit I quoted was basically the whole thing here, I wish I could find the one I'm thinking of!
The "focus on the family" style punishment focuses on telling the child that you're doing it for their own good, that you're purging their badness through pain, and focuses only on ritualized spanking on the buttocks.
Moving corporal punishment from a thing done in anger - like cuffing a child in anger (which is still awful) - to a thing done calmly, for "love" and "holy purpose" twists it into a whole other thing. There's a lot of weirdness with shame, nudity or forced disrobing, and pain that gets dredged up there.