I mainly avoid Psychology Today for its frequent reckless promotion of retrograde pop psych, but this article on S&M is pretty good:
Bind my ankles with your white cotton rope so I cannot walk. Bind my wrists so I cannot push you away. Place me on the bed and wrap your rope tighter around my skin so it grips my flesh. Now I know that struggle is useless, that I must lie here and submit to your mouth and tongue and teeth, your hands and words and whims. I exist only as your object. Exposed.
Of every 10 people who reads these words, one or more has experimented with sadomasochism (S & M), which is most popular among educated, middle- and upper-middle-class men and women, according to psychologists and ethnographers who have studied the phenomenon. Charles Moser, Ph.D., M.D., of the Institute for Advanced Study of Human Sexuality in San Francisco, has researched S & M to learn the motivation behind it--to understand why in the world people would ask to be bound, whipped and flogged. The reasons are as surprising as they are varied.
The article provides ammunition for those who accept consensual S&M as a healthy behavior and those who consider it sick, which makes it fifty percent better than most.
My personal opinion is that all sex involves the expression of power and helplessness, so S&M just makes it explicit. Someone is always pursuing, someone is always being pursued; someone is doing the fucking, someone is getting fucked. This dynamic is present, I think, even if it's not noticeable from the sweet, romantic exterior. (In our romantic myths, there is a reason damsels are in distress - distress is hot.)
I realize a lot of people would disagree with this, mainly on the grounds that it seems to undermine the loving aspect of sex. But - and the article doesn't do a very good job of showing this - kinky sex is loving sex, too, or at least as with all sex it's as loving as you and your partner are. There are many people who enjoy kinky sex but refuse - sometimes angrily - to identify it as such; they prefer to think it's their own thing, special, not subject to categorization. Partly it's out of the negative connotations around "whips and chains", but it's also the consequence of sex being something Americans permit themselves to obsess over but not think about. That latter tendency is ever so slowly beginning to diminish, which is good, but we have a long way to go.
There is also a strain of feminist thought which holds that kinky sex involving a man dominating a woman is abusive, patriarchal and fucked up. I try to engage these ideas by pointing out that there are quite a few dominant women out there, and that kinky porn is as often about (you know these links are NSFW, right?) men submitting as it is women. I think the problem here is that so much mainstream porn is outright misogynist that the presumption is that kinky porn is, too. And it's also true that hardcore S&M uses elements of misogynistic and homophobic culture, since you have to use what the culture hands you. I argue that intentionally making those things part of play actually disempowers them, and enables both the top/dom and sub/bottom to recognize and fend them off outside of the "scene." Clearly, this discussion is beyond the scope of most people's interests, and that's probably a good thing.