Spanking with a belt is considered child abuse these days, but using just your hand is not considered abuse by many parents. But if you did the same to another adult it would be assault. Yeah that makes sense.
On the other hand, if your kids are being utter brats and are making your life miserable, especially if they're doing it on purpose, you tend to resort to your most primitive of instincts. Or you fantasize about doing it. And they don't have to be your kids but you know you can't even touch them, so...
I heard some people get turned on by spanking. I once saw a lady admit it on Dr. Phil, only I didn't actually "see" her, I only heard her speaking because she wanted her identity hidden. But it can't be that taboo because I once saw it on a "dad from daughter" card while shopping for either Father's Day or my dad's birthday. It showed a guy with his wife over his knee in their bedroom when their young daughter comes in. Not realizing this is just how they make love, the daughter says "just give her a timeout, Dad!" No, I did not get the card. Like many people I'd rather not know what goes on in my parents' bedroom.
I read that in Middle Eastern countries husbands spank their wives, but as punishment.