BDSM players negotiate an agreement about the structure, guidelines, rules and boundaries of a scene or relationship. This is understood as a contract (though not legal) emphasizing the importance of consent, to ensure the safety and confidence of participants.
Kinksters say requiring consent ensures trust. But ethicist Sarah Lucia Hoagland observes that “physically and emotionally submitting to another” is not the same thing as trust.
Moreover giving consent in sex is not a simple act. Judith Butler describes it from her experience as a lesbian feminist — but I think this would apply to S/M in general:
The task of learning how and why we consent… is a constant struggle. Sometimes we say yes, not wanting to, and other times we say no when our insides are screaming to say yes. But most times our desires are not so straightforward. They are, I think, complexes of things, fears, hopes, memories, anticipations. They arise from our concrete situation and are colored by the ambiguity of our experience. Consent can only be as complex as desire itself: consent is not a simple act, but a project, a skill we have constantly to learn.
Claudia Card points out that contracts require
• the parties be adequately informed
• their consent be free (uncoerced)
–and then considers the possibility of addiction to BDSM, where both those conditions could fail:
• “The parties would have to be aware … of the likelihood of addiction, information not readily available in a society that does not support dissemination of information about sexual sadomasochism.”
• “Addictions diminish freedom to withdraw…[suggesting] that over time, what began as a consensual activity may cease to be clearly consensual, not because of communication problems but because of diminished responsible agency on the part of either or both parties.”
Card lists precautions that should be included in BDSM contracts — but questions whether they are:
Some classic activities of sadomasochism, such as bondage, do curb agency, not simply control, at least temporarily. Being tied and gagged with a hood over one’s head severely impedes communication. A clear safe word should be more specific than a groan or a wiggle. To prevent fatal accidents and keep open the possibility of communication, a sadomasochist agreement should also include advance safety understandings, such as that no one will be left tied up unattended or left face down in soft material that might hamper breathing. Scrupulous M’s are expected to inform S’s in advance of disabilities, disorders, or vulnerabilities, such as allergies, diabetes, or heart trouble. These precautions are taken from an essay on safety in Coming to Power, whose author insists that such advice is only common sense.For those seeking the thrill of danger, however, that may not be obvious. Concern for basic safety and keeping open the lines of communication may compete with the pleasures sought. Constant higher100 order communication may disrupt the spirit of play, intruding too much reality.All that is clear is that if higher order consent is to be continuously consulted, such precautions set limits. What is not clear is that consensual sadomasochists will be interested in continuous consultation.
Women’s studies pioneer Sally Roesch Wagner comments that true consent requires that neither party be dependent upon the other: “Consent implies the power to say no.”
Maryel Norris (a contributor to Against Masochism) elaborates:
There exists no hope of “choice” in a situation where one becomes lovers with someone in a prescribed dominance relationship. The partners are patently unequal and no matter how much care is taken, the issues of dominance and power remain. One may seem to be choosing but, instead, be succumbing to the force of an overwhelming socialization for passivity and for playing the victim.
Pioneering expert on sexual violence Diana E. H. Russell points out that
women have been reared to be submissive, to anticipate and even want domination by men. But wanting or consenting to domination and humiliation does not make it nonoppressive. It merely demonstrates how deep and profound the oppression is.
Card agrees: “Consent … does not nullify oppression. And consent of the oppressed is, presumably, not freely given.” 
Indeed, in light of cultural conditioning, consent looks to me rather irrelevant. I think of children subject to the will of parents: they can give no true consent to incest. A woman staying with her abuser out of fear of reprisal, or out of lack of alternatives. Nineteenth-century young Brahmin widows who “voluntarily” jumped into their husbands’ funeral pyres. All are victims of cultural conditioning. 
John Stoltenberg explains who in our culture actually has the power to consent:
“Consent” presumes that both parties to an agreement are equally free to make the agreement, have the same actual freedom to agree or disagree, and have the same actual latitudes of actions, opinions, or sentiments from which to choose. “Consent,” therefore, is a concept that only has meaning between two persons who are equally enfranchised by culture to act willfully and without constraint—people, that is, who are genital males.
 Hoagland, in eds. Linden et al., 157.
 Butler, in eds. Linden et al., 172-173.
 Card, 1995, 232.
 Janet Bellwether in ed. Samois 1982, 69-79/a>.
 “Higher order”: This refers to Card’s concept of levels — “orders” — of responsibility, power and choice. A higher order encompasses, controls and takes precedence over a lower order. See kaybaird.com/index.php/part-i-it-begins/chapter-4-the-consequences/am-i-a-masochist/”>Am I a Masochist? /The Power Dynamic.
 Card, 1995, 227-8.
 Wagner, in eds. Linden et al., 30-31.
 Norris, in eds. Linden et al., 106-107.
 Russell, in eds. Linden et al., 177.
 Card, 227.
 Only since 1993 has wife rape been a crime in all 50 states of the U.S. And the assumption that wives owe sex to husbands still persists in laws that make it harder for a wife to accuse her husband of rape than it is for an unmarried woman to accuse her rapist. See Diana E. H. Russell’s book Rape in Marriage. And see 23 states make it harder for a wife to accuse husband of rape.
 Stoltenberg, in eds. Linden et al., 127-128.