One of the paradoxes of SM revolves around consensuality. Everything in SM is consensual; although the top is in control, it was the bottom’s choice to allow the top to _be_ in control. And since most of the time the top is trying to keep the bottom happy, and since the bottom knows that, isn’t the bottom really in control?
Yes, if your definition of “in control” is “can stop the scene”. The bottom can always opt out, if it’s consensual SM. But the top can go a long way towards putting the bottom under the top’s spell, making the bottom submit to really strenuous bondages or beatings, using the bottom as the top pleases. One friend of mine, for instance, takes great pleasure out of hog-tying his girlfriend with her breasts bound and her hands behind her back and her ankles tied to her ponytail, then putting her on the edge of the bed and sticking his cock in her mouth. She has no choice but to suck it until he comes. Which of them is “in control”? Both of them would say that he is, and both of them are getting off on that fact, so the paradox in practice doesn’t matter too much.
Furthermore, negotiation can involve give-and-take; the bottom can agree to endure some pain to please the top, and the top can then (say) tie the bottom tightly and tease to the point of orgasm. A particular activity in SM can be enjoyable for its own sake, or because it’s a favorite sensation of yours, or because it turns your partner on so much to do it to you or with you, or because you want to endure it out of pure stubborn pride. The paradox of control can take many forms.
As mentioned previously, some especially intense players may negotiate scenes during which the bottom _cannot_ opt out. This sort of play is definitely in the minority, but it is nonetheless possible to consent to giving up your ability to withdraw consent. These scenes are sharply bounded by mutual agreement between both partners, and must be accompanied by much discussion, before and after the scene. And if the bottom later feels like the scene went bad, the negotiation should be redoubled before the next scene, if any.