Heh... This is a topic I want to jump in to, seeing as sometimes we realize things about ourselves that shock us.
Domination/submission is often thought of in terms of torture like the acts referenced above, but most people don't realize that it can be much more subtle and EXTREMELY erotic. Yes, I enjoy the thought of "disciplining" a guy, but what's really fun is verbally dominating him, making it known that he is your toy, and making him LIKE it.
It's a kind of balancing act, of sorts, really, Domination and submission. Everyone involved has to clearly define their boundaries and be at peace with themselves, and there should be a safe word for reasons listed above.
Everything exists on a continuum, and D/s is no exception.
As for legality, where the line is drawn... I think that if the safe word is not respected, then the line has been crossed. If they don't say the safe word, it was consensual. As for any "bizarrity"... I'm still thinking heterosexual intercourse is unnatural and wrong.
But I recognize my own biases lol