Byakuya is aware that this is a dream, and that is the only reason he tolerates it. If it wasn't a dream, he would have fought back. He could have fought back. He would not accept such intrusive behaviour, such offensive liberties --
"Of course I understand the needs of the people below me," he'd told the other Captains as they sat down together for tea and discussion. "But they shouldn't see their Captain as fallible. To confess weakness can only weaken the Division. Not that I mean that with respect to anyone here . . ."
It's a dream that he has been forced to his knees, that a blade was put to his throat, that his hands were bound behind his back with his own scarf. It's a dream that he can be bent over his own desk -- his neat desk, his tidy desk, his perfectly clean desk -- and feel those large hands on his bare flesh.
It's a dream that he can accept all this and be weak and be helpless and that nobody will know about any of it.
". . . and of course I accept that sometimes a Captain has to speak to someone, but in that case he should choose his confidants carefully."
It must be a dream, it can only be a dream, because otherwise his mind would not toy with such ugly, lower-class words as he is taken on his own desk. Fucked. Screwed. The sheer vulgarity of it is a frisson. The concept is something that he would not have chosen to imagine. So it can't be anything other than a dream.
It's a good dream.
It's a dream that makes the breath catch in his throat as he hears himself moaning, little gasps for air, whimpers, noises of submission as Aizen (and there, it must be a dream, because Aizen would certainly never do anything like this) takes him and he is helpless to prevent it, Aizen's hand circles his manhood and controls him, Aizen uses him and he submits and he doesn't want to do anything other than submit.
So it must be a dream.
He drank from his cup of tea, poised and controlled as always, and looked around at the other Captains for some hint of approval. Unohana smiled gently. Ukitake nodded, but with a shadow of reservation. Aizen smiled gently.
It's a dream that doesn't stop too soon. It goes on well into the night, as Aizen takes him over his desk, then makes him serve on his knees, then sits in Byakuya's chair and takes him face to face, and Byakuya buries his face in Aizen's shoulder as he begs for it to go on and not to stop, because there are no limits here, no boundaries, no pride. He has gone past his family pride and his personal pride and even his pride as a shinigami. He has been reduced to this.
It's a dream that he wakes from in a blur of images that he then chooses to put away and try to forget, because a Captain does not do such things, an aristocrat does not do such things, and he himself?
He would never do such things.
And certainly not with someone as gentle and mild as Aizen-taichou.