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You see, Dean has this thing for Castiel's wings. Can't see them with the naked human eye, no, but those glasses scorched with holy fire? The ones that allowed him to see the hell-hounds? Yeah, they allow him to see Cas's wings. Freaked him out at first, because, for some reason, he wasn't expecting them. But ever since then he can't keep the glasses off his face for very long once Cas has his back turned or isn't paying attention to Dean.
Dean often finds himself jerking off to those gorgeous black wings. He likes to imagine what they would feel like caressing his cock, how they would feel against his sensitive nipples. Would the feathers be soft or would they be coarse and scratchy? He likes to imagine it both ways.
Dean didn't even know if he was attracted to just the wings or if he was attracted to both wings and vessel alike. He was afraid to look too deep and know.
But Cas, well, Cas has always known about Dean's wing kink. From the very first time Dean saw them, Cas has known. He can hear every prayer, especially of those in close proximity and Dean's thought was simply, "Dear God, that's fucking hot." Now, while not really a prayer, it still contained the words Dear God, so it catches on angel ears period. It can get very obnoxious, which is why most angels don't answer prayers anymore. Too many people using His name in vain. But this one caught Castiel's attention like nothing else did.
So, now, Cas takes every opportunity to turn his back to Dean, knowing he has the glasses on him at all times, and he spreads his wings wide, arching them and making them just flat gorgeous. At night, Cas makes himself invisible to human eyes and spies on Dean jerking off to the image of his wings. Cas always knows when he favorite human is jerking off because their Special Bond keeps him more in tune with Dean's prayers, and like most humans he uses the oh god whenever he really gets going. He even uses Cas's name, though Cas is sure he doesn't even realize it. The humans have a word for what Cas is, and he looked it up on the internet once to see if it was an actual thing, and he is a voyeur through and through. He likes to watch, but as an angel of God he can not participate lest he break even more laws among his kind. He's broken enough, and fornication with a human, male or not, is strictly forbidden, a line is not willing to cross. Yet.