The one difference we use in defending BDSM being tantamount to abuse in vanilla courts, both those of law and public, is consent.

Now so long as whatever is happening is happening between consenting parties (no matter how sick, perverted yada yada yada is in someones opinion) is your private thing.

But when one of the parties is incapable of giving consent...we are talking about rape and criminal behaviour.

Animals, children, corpses....I am sorry, but that is morally indefensible.