Isn't Spanking a forum of Sex abuse? Especially when.. it has to do with the hand being next to the genitals down there? Even if the power of the beating is on the cheeks of the buttocks.
If consensual, it depends on how hard it is done. Inflicting anything other than minor pain is unwise as it leads to injury. Common sense. If someone wants to be whipped, for example, it's abuse whether it's consensual or not.
If someone gives you consent to nearly chop their fingers off, even if the consent is written, do you think it's ethical to do it? This is an extreme example, of course, but these are things to think about.
Consent will not magically delete the consequences of anything. If someone likes scat, vomit or otherwise playing with bodily fluids, which is unhygienic, they'll get I'll from it. It's common sense.
There's no need to ask other people's opinion about anything sex-related. Just think about it rationally. What are the consequences of doing this? Have I researched them? If there's no harmful consequences, that's okay.
Let me make another example. Some people like their hair to be pulled. Doing gently is obviously inconsequential in terms of harm, so it's alright to go. If you pull too hard and cause a lot of pain or even rip a chunk off, then that's not okay.
Some people like hickeys, which can be alright up to a point. If you're making your partner bleed from it, you're definitely going way overboard.
Common sense.