On the face of it, being bound is humbling since the unbound party is assigned the manifest power in the equation. The way in which we perverts temper the politically awkward fact of a power gradient obtaining between two parties is to call it a "power exchange", but there's no getting around it: one person humbles, and hence humiliates, another. What I would like to counterintuitively assert here is that a humbled state is about as close to the possession of pure power as any person can hope to achieve in life.
It has always been possible to subvert the will of a presumptive controlling party by creatively appropriating the presumed (sometimes ceded) mechanism of control. Thus, for example, has the word "queer" lost all of it's force as invective. Consciously surrendering to humiliation and degradation, to being apparently reduced and controlled by another, is the difference between being empowered and being oppressed. This idea is both the fulcrum and the lever of consent, and like those two basic machines there is practically nothing that consent cannot move.
All mystical traditions recognize that oppression is an optional state. Christ did nothing to resist the cross; in fact he actively sought the harshest of judgments from Pontius Pilate and the Pharisees. He was not oppressed in the slightest - rather, he was impassioned, humble and went down willingly to the grossest of abasements. He loved his enemies (and I have to doubt he thought of them as enemies). Had Christ indeed permitted himself to be oppressed by his oppressors he would then have likely have been forgotten along with every other Jewish carpenter named Jesus from the Galilee of his era. That he (as the tale is told) gave himself, that Christ surrendered, is what is remarkable about the man. A few hundred years before Jesus became the Christ, Siddhartha Gautama was born a prince, surrendered to humility and became the Buddha. Among the liberated community of our own era, the stories of Nelson Mandela, Václav Havel, Aung San Suu Kyi and countless others bear out the maxim that oppression is optional.
It's a bit more difficult for me to come up with an argument for oppressive practices other than bondage (say, whipping) being transformative in the direction of liberation (maybe I'll have a go at that sometime - the flogging scene in the movie Glory springs to mind), but I don't doubt that the same principle applies. Tying someone up looks to a tyrant like a ready means of reducing what it means to be a fully realized human, so the tyrannical community reliably adopts physical restraint as one of its tools. The liberated community knows in its bones that such behaviors are gestural, impotent and ultimately futile.
Given, however, that consciousness first blinks on in a monadic cosmos where all data points to our being all and one with Everything, that is, in a flawless state of union with the principle of creation, the perversion of physical restraint into eroticism is a small leap. Between the bottom of our hearts and the backs of our minds we already know what our deepest nature is and that it is continuous with the whole of existence. We come to consciousness both enlightened and tightly constrained, our fledgling senses accommodating only the toroidal nature of the womb. It is a comfort, a state of knowing without thinking, from which Freud observes we part only reluctantly. An atavistic impulse to return to the perfect state of satiety and unity is associated in earliest consciousness with being in a tight spot, understood as such only a posteriori, and certainly distinguishable from the open and vulnerable physical and psychic space we come to view as our world.
A commonplace about enlightenment is that separation and individuation are finally understood to be falsehoods, and so the inevitability of death is cast in a wholly different light, one unpacked of fearsome meaning (or any meaning at all, actually). To really live then becomes what Kant, in his consideration of the Sublime, liked to call Zweckmaßigkeit ohne Zweck, (purposeiveness without purpose). Alfred North Whitehead noted in his Function of Reason that the precise purpose of authentic inquiry is to be purposeless. At the edge of experience purpose is understood by scientist and mystic alike to be a mental, egoic construct, one which I suggest endows the entire notion of oppression with its noxious cast.
Indeed, to be enlightened is to fear no evil.
All ecstatic experience (or, again, passion) points to ultimate principles, and orgasm is the one form of ecstasy with which most people will have at least some experience in their lives. We partner-up intending to bridge the divide between at least two individuals, and fucking is largely (and merrily) how we prosecute that intention. So, as a species on at least one thing we're all in agreement: deep relations with at least one other person = good. Extrapolating only slightly from there it's easy enough to infer that deep relations with the whole of creation = even better. Getting sexed-up leads to ecstasy leads to reconnecting for a moment with the Godhead we know from earliest consciousness. Communion with ultimate principles is bred in our bones. Christ, Ghandi, Lao Tzu and nearly everyone else with their eye on the ball tell us more or less the same thing: We are God, we are All, what we think separates us from our true selves is an illusion. Gestation and birth are just metaphor for the state of grace and the fall from it.
(If we already have grace and know the ultimate truth, why bother with getting born and having duality, individuated self-hood, identity and all the other head-fakes of waking life in the first place? What's the advantage? Is the Universe in the business of squandering perfectly good creative energy?
The appearances of life have their own grade of cosmic import and are yet another topic to be considered another time.)
The physical constriction of bondage is a ready return to the antecedent state of grace. In getting tied up under a consensual, surrendered framework it's relatively easy to transform the intention of someone interested in controlling your freedom into a free ride to satori. Doing that allows the power gradient to shift immediately in the direction opposite to what is normally assumed, and tyrants need not apply.