Here is a handy explanation of what radfems think of bdsm:

We argue that BDSM is a sexual practice that hinges on degrading, humiliating, violating, and torturing people (usually women but it is unacceptable in any form). When performed against women it is a mirror of patriarchical violence against women which basically happens everywhere already, only now it is being praised and sexualized.

We don’t blame women who participate because women are raised and socialized in a system that encourages us to view ourselves as passive recipients, submissive, or even encourages us to view violence as sexuality. This socialization runs deep and begins from the time we are small children (listen to the discourse when a 5 year old boy hits a 5 year old girl, “he likes you!” “boys will be boys!”)

We view bdsm as the legitimization of violence against women.
Part of the problem is that no one can truely consent to violence done against them. We believe that the line for “she consented” being an acceptable answer is where violence begins. This has both legal and social implications.

On the legal side, making bdsm illegal would not really affect what individuals do in their private bedroom, there is no precedent for watching that and radfems don’t want that anyway. It would, however, allow rape victims or abuse victims to file a complaint against their attacker without worrying that he is going to tell everyone that she was okay with it. This is not something that only happens in fairly “vanilla” bdsm situations either. There have been cases of people eating one another and claiming consentuality, people have “consented” to being murdered (I am not talking assisted suicide here) by a man who has to strangle people to have sexual release, and consented to being tortured and strangled to death. These are things that radfems think can not be adequately consented to. There is no situation in which violence against another person should be acceptable.

On the social side of things, a public outcry against BDSM would allow for women to come forward about their abuse with at least slightly more encouraging results. As it stands, if a woman says she was tied up and raped she has to prove she didn’t want that. If there was a public outcry against the idea that violence can be consented to that problem would at least partially go away.

Some of the criticisms against radical feminists beliefs on bdsm include that we are sex negative, prude, frigid, or “slut shaming” (even though we also advocate for the recognition that no woman is a slut and actively decry the term “slut shaming”). We are told that we don’t respect the agency of women who choose to participate in BDSM (even though we never blame the women who participate in BDSM, we shame men who dominate women, not women who for whatever reason, choose/are made to participate in an abusive subculture). By blaming men we are called kink shamers (a title most of us gladly wear if it is code for “shaming men who get off to beating and raping women”).

Many radical feminists speak out against bdsm because of the abuse and subsequent silencing of our abuse by the bdsm community. We, and other women who have spoken out, are told that we should have sad the safeword or that our experience is just ours and reflects nothing about the community, or even that we weren’t REALLY in the community, that we were with an abuser, not a real dom, etc.

From Piscula