snowshapes - you raise a lot of the points that many people outside the BDSM scene raise, I'll try and explain some of them for you.
Consent/the power being with the sub:
This is very much analogous to sex in a 'traditional' relationship where no means no, except it is more likely to be red/trainspotter/almond for example that means stop. The bottom line is that a Dom(me) who doesn't stop at this point is assaulting their partner just as much as a person who ignores no/stop etc in a none BDSM relationship and is considered by their peers to be just as reprehensible.
BDSM seems to be about violence and pain, about humiliation and domination of another person:
Domination yes, the rest? Not necessarily. A popular misconception about BDSM is that it's about pain/always involves pain, this is far from true. Some practitioners are very much into the bondage side, be that handing control of their body to their Dom(me) or taking that control. For many, being bound gives them a release - a sense of freedom if you like - they trust their partner not to hurt them. Others are into pain - giving or receiving - and enjoy the sensations. The key is safe, sane and consensual. For a lot of Dom(me)s it's not about getting off on causing pain, it's about being able to control with absolute precision what their sub is experiencing. A good Dom(me) is generally incredibly protective of their sub, extremely considerate of their welfare and keen to encourage their sub to develop as a person. A healthy D/s relationship brings the best out of both parties. Where the major problems come is when you have a pure sadist pretending to be (or who has convinced him/herself that they are) a Dom(me)...that never ends well.
Gender breakdown of encounters:
I have no idea what the stats are, I do know that I've seen every combination possible...many times. I'd say I've probably seen more male/male pairings than any other, but it would be close.
Equality:
Some people are naturally dominant, some are naturally submissive. A common mistake is to think that submissive automatically equals weak, it doesn't. Everyone is different, people have different needs and desires and that should be accepted. There are many BDSM practitioners who are in high powered roles professionally, but seek an escape from decisions at home. Not everyone WANTS a 50:50 relationship, surely they should be able to seek the relationship they want/need as long as their partner is freely consenting? What there does need to be - especially where cases come to court - is a greater understanding of the BDSM world. Unfortunately, much like rape within marriage/a long standing relationship or dv, you're often in the realm of he said/she said.
Maybe I'm struggling to separate BDSM 'done right' from the wider degradation and objectification of women in mainstream porn/erotic writing,
You wouldn't be the first (or the last).
I hope my ramble has helped a bit.