It starts as a thought lingering in Dean's mind, a single thought that seeps into his dreams and manages to become a fantasy. (He shouldn't say a because that implies singular and he's had multiple fantasies about Cas; they usually involve handcuffs and doing shameless things to the angel's wings.)
Castiel finds out by accident, attempting to wake Dean from a nightmare only to discover it's quite the opposite. He views Dean's dreams simply to make sure it isn't a bad dream. What he finds there is . . .surprising (he finds a dream version of himself nude beneath Dean, legs identical to his wrapped around the human's waist); the angel moves without thinking. Suddenly, he's halfway across the globe, alone on a mountaintop.
What a curious development.
He understands that humans have basic, primal urges and that sex is included among these, but he never would have thought that he would be the basis of Dean's focus. Still, Castiel knows what is expected of him and he confronts Dean immediately(away from Sam because he knows Dean values his privacy and would find this extremely embarrassing).
Dean, at first, resists nobly, but eventually gives in and kisses the angel. Castiel does not tell Dean that he won't feel any of it; the benefit (or curse) of being an angel is that one doesn't experience human needs or emotions at least not to the extent that actual humans do. But, like a martyr, he bears it in silence as Dean caresses him and lays gentle kisses along his jaw.
He feels something, a sensation, burning under his skin, but he ignores it as Dean strips off their clothes and kisses his lips tenderly. The feeling makes him furious for some reason though he makes sure to keep his face blank as Dean slips his fingers into the angel's body and stretches him open(he wants to tell Dean to stop trying to make him feel it, that he can't and won't-).
Then, the sensation, at first a buzz, now erupts and starts to sear his flesh; Castiel is sinking down on Dean's cock, trying to ignore it when his grace begins to sing. He had placed a piece of his grace in Dean's soul when he rebuilt the human, but now it's as though Dean's soul and Castiel's grace are calling to each other. Pleasure consumes his body as Dean repeatedly strikes his prostate, the human kissing his throat.
No. Castiel thinks as he feels his human body react to Dean's touches. This wasn't supposed to happen. He briefly wonders what this will mean for them and then tries to not scream as he feels his body tightening; the angel buries his face in Dean's shoulder, trying futilely to keep his grace from bonding even further to Dean's soul when it happens.
His hand seals over the mark on Dean's shoulder and almost in unison, Dean and Castiel release, their graces meeting and if not for their closed eyes, they would have been blinded. The angel is almost thrown from his human vessel and his grip tightens on Dean's flesh, leaving bruises.
When they finally come down, Castiel raises his head and he can feel the bond cementing between them; he meets Dean's eyes, praying that he doesn't feel it. Dean musn't because he leans forward to kiss the angel, but as he does, the bond flares even stronger in Castiel's mind and he runs, spiriting himself away. Naked and shameful, he sits on the mountaintop, with tears in his eyes, blaming himself for this thing he should have seen coming.
Cas. . .