My brother's bible class was teaching him about "addiction"

[Esquilax]

Well of course there's a legal side - and most people do not like the idea that people are "voluntarily" filling up hospitals. So legally of course it makes sense that causing another person harm is always wrong.

I'm getting the feeling, from this, that you don't have a lot of experience with bdsm. That's fine, but I do need to point out that you're severely overstressing the case by imagining this as the kind of harm one goes to the hospital for. There are bruises, yes, but rarely more harsh than what you'd get from a punch on the arm. I'm not going to sit here and pretend no kink related injuries ever happen, but responsible kinksters understand the risks and account for them.

Within the context of the law, I think a case could be made that the line needs to be drawn a little tighter than "harm" in general. Is spanking okay? What about sex toys? I've seen a lot more emergency room trips for those things getting stuck in private places than I have bdsm hospitalizations, after all.

As Chad very rightly pointed out, we sometimes allow ourselves to undergo low impact harm for some other purpose all the time. In the case of bdsm, we take the pain because we're wired in a way that allows us to derive pleasure from it, or because of the endorphin rush, or the headspace one gets into during. Done correctly, it isn't harm that costs anyone anything; bruises heal, marks fade, and they don't impede your life or require outside medical treatment in the interim. Should there be laws preventing serious self inflicted injuries? Sure. But if one makes a cost/benefit analysis of kink, I doubt one could come to the conclusion that the costs are even really present, above and beyond the normal background risks that everyday life poses.

Now you missed my point - the type of activity depicted in Kink videos is legally grey at best, thus if it's legally grey in the first place it shouldn't be legally OK as acceptable pornography.

So where do you draw the line, and why? If a girl gets spanked in a porno, is that over the line? What if the guy uses an implement, and not his hand? How red does her ass need to get before it turns a shade of legal grey?

There are obvious signs of harm that are clear indicators, and there are other, more subtle variations. I would argue that Kink does rough movies, but in their work I've never seen more than redness of the skin. The irony here is that before Kink came along it used to be far worse; there were companies preceding them that were absolutely brutal, ones that really did ignore safewords and consent, shit that I'm glad went away. I go to Kink precisely because nobody breaks the skin, and I don't have to see black bruises the size of my hand.