A dear friend wrote to me about feeling that vanilla sex was inadequate and though he’s not particularly oriented towards BDSM, he’s decided to start exploring it to expand his sexual repetoire. He writes:
From my limited experience:

Pain is different from harm. Whether the two coincide often has to do with intention or context–the pain felt during childbirth is processed and experienced differently (and usually not as psychic or emotional harm, even if the body is injured) whereas an equal amount of physical pain felt when being tortured in someone’s basement or in a POW camp I would guess is much more likely to be harmful.
In less dramatic senses, I think there is more risk of harm when we are acting out of any sort of malice, retributive anger, hatred, resentment, defensiveness, aggression–anything that Yoda would put on the dark side of the force.
This purtains to kink in that, when it is done in a “safe, sane, consensual” manner, one of the intentions may be to cause pain–physical or emotional–but not to harm. Anyone who wants to do you lasting damage isn’t someone that it would be healthy for most people to get kinky with. A good top, when you are not “playing” with power or other kink stuff but doing something more serious, is more like a spiritual guide, knowing when to push and when to slow down, open to feedback, offering support and encouragement when necessary, with steady love beneath whatever else is going on or whatever the expression of it is (you know I don’t necessarily mean romantic love). And in this context, the sub has the opportunity to experience and confront some of the things we struggle most with as humans– perhaps, physical pain, which she will learn is easiest when one surrendors to it or accepts it wholely; mentally,
humiliation–which can be an joyful release from selfhood, an entry into intense trance states, a way of taking the ego and breaking it against a rock, failure or guilt–which, in going into fully in the safety of the setting she will learn to fear less in daily life, and to meet fully when it arises, her psychic and physical limitiations–which may help release her from some of the perfectionism conditioned into us by our culture, fear–which she will become intimate with and learn to and enjoy, create for herself the tool of imbuing the terrible with the erotic thus helping her to face it, to make it bearable.
It is a grounds, perhaps most of all, for giving and recieving unconditional love. There is incredible risk on both sides to exposing “shadow” sides, in asking for obedience or giving it, in giving a command or following it. The scene can exist only when both parties conspire together, are in it together. And it is amazing, to humiliate oneself completely in front of someone, to for a period of time exist in a state of utter trust and let someone cause you pain without trying to escape, and instead of leaving, the person stays, appreciates, loves you all the more. And the top, I would guess, has a reciprocal experience–to demand, inflict, command, humiliate, and still be loved. It’s breath-taking, isn’t it?
The difference between this and actual abuse has much to do with explicit consent and intention. Abuse often comes from intentions to harm, defend, protect, intimidate. Kink, in good situations, comes from intentions to expand and open emotionally and experientially, to achieve intimacy, to give and recieve love, and often includes inflicting pain in the service of these things.
Of course some people use kink to channel hatred of various sorts, or to put themselves in harm’s (rather than pain’s) way, and in those scenarios, there is great potential to damage all involved. But in the best cases, sex can become a pretext, a means, a background, or simply a component of a deeply intimate, alchemical process.