It says a lot about Sam that this isn't the unhealthiest relationship he's ever had with someone.
Oh, it's up there. If he needs another tally on his endless track record of terrible decisions, then he can jot this one down without batting an eye. It's not even the 'screwing a seraph six ways from Sunday' that leaves him feeling like he's done something unforgivable. It's fucking Cas, and knowing why Castiel lets him, and knowing, that despite both of those things, Sam isn't going to stop. He hates himself, but he's not going to stop for however long he can keep doing this because the alternative is so much worse.
He doesn't ask Cas why he's waiting on the end of Sam's bed. He doesn't ask the more important question either, the one that hangs over them like the executioner's noose every time they do this. Dean's bed is cold and empty, and he isn't coming back tonight. Sam is nothing if not an enabler of the fucked up and fucked over.
Cas likes to kiss him. He has to wonder if that's pity for him or just another part of the fantasy. If Cas thinks he's taking advantage of Sam as much as Sam is of him. Maybe he is and Sam's getting the short end of the stick here, but it really doesn't feel like it. Sam kisses back anyway. He's got an ache somewhere in him that Cas can't fill, not exactly, but enough of the edges line up that he can force the piece into the puzzle. If the cost of that is Cas holding his face like he is something precious, then Sam can deal. Cas kisses him, and Sam can taste an angel.
It's not as strong as it once was, when they first started this. At first, Sam had thought he'd been imagining it. The second, and scarier, thought he'd had was that he was developing a tolerance. Every time they come together, that spark in Cas is a little weaker. His eyes are more tired at the edges, and his reactions get more intense each night. In the end, it isn't a fault with Sam at all. He can't even claim the guilt of being the one to make Cas fall. He's just the stops along the way, and eventually, there's going to be no more angel left in Cas. Sam's not sure what he will do then.
He could, his brain tells him traitorously, get his fix somewhere else. Somewhere in dreams he tells Dean are torture, where a hand caresses his chest and rests over his too-calm heart and he hears again, you were made for me.
No, he thinks, I'm not. It's easy when Lucifer isn't there.
Cas lets Sam push him down. He's still strong enough that he could throw Sam off, if he wanted. The knowledge is a cold comfort when he knows Cas never will. Instead, Cas watches him undress. His eyes linger on Sam's tattoo. It's easier than meeting Sam's eyes, and Sam is grateful for that. Cas- Cas's vessel- Jimmy. His eyes were blue, but not the right shade, so close and so far. He as desperately wants the reminder of who he's replacing when he leans over to get Cas's pants off as he wants to never think about Lucifer ever again.
Sam is already bad at that, and he's getting worse.
He doesn't even bother with Cas's shirt, just leaves it on. It's not the body under him that he's interested in. It's the too-sharp brightness he knows is under Cas's skin, and the unnatural strength that will leave bruises on Sam for a week, and the reverence Cas has when he touches Sam. It's not even meant for him. He's the wrong Winchester. Cas has the decency to not say his brother's name in bed. Sam does the same and succeeds mostly because he's still not sure what counts as a prayer or not.
He fumbles with the lube. The first few times, Castiel used to get frustrated, prepare himself with his grace. He doesn't do that anymore. Not worth the waste. Or maybe he thinks Dean would prefer it like that, working him open slowly. Sam can do that for him. The sounds Cas makes would be glorious if Sam loved him. One day, if they still have a future, this is going to end, and maybe Cas will admit everything to Dean. If he does, Dean better cherish Cas as much as Cas will him.
If Sam were a better person, he would have stopped this before it even began and pushed Cas and Dean together to live whatever happily ever after they could have at the end of the world.
Sam is not a better person. He pulls his fingers out of Cas, puts one of Cas's legs on his shoulder for leverage, and thrusts.
It doesn't take him long to find the right angle, the right speed, because by now, he knows what Cas likes. Cas is warm, too warm, below and around him, but Sam's got a good imagination and a lot of practice at denial. He's not rough with Cas. Not half as rough as he wants to be. (As Lucifer would let him be, the same way he lets Sam scream and rage at him, smiling like Sam's anger is something beautiful.) It isn't a part of Cas's fantasy, and what Cas wants comes before what Sam does. If it'll keep him here for even one night longer, then Sam can be as gentle as Castiel imagines Dean to be.
Sam fucks him. He closes his eyes. Cas keeps his open. Cas's hands touch the tattoo Dean shares. Cas carved the same symbols into their ribs. Down in his bones, Sam is close enough to Dean that his soul doesn't matter. If he wasn't wondering what Cas's blood would taste like, if it would give him what he needs, then he'd probably care. He's not that far gone, not yet. Before the end, though... Who knows. Whatever it takes to keep him here, to keep them both here, so that Sam doesn't end up saying-
Cas grabs his shoulder hard. The bruises he left the last time they did this haven't completely faded yet. It hurts. The pain is almost freezing. Cas is coming with a gasp, and he's not saying, "Dean," but he's sure as hell not seeing Sam either.
Sam bites his throat. Keeps himself quiet as long as he can. Which isn't very long.
"Yes," he chokes out against Castiel's skin, where it will be smothered and useless. "Yes," and there is the crux of Sam's damnation. He shivers and lets himself think only for a moment about Lucifer's promises, destruction and worship if Sam will only let him, and he wants. He wants like he is going to die. He pours that wanting into Cas, takes all that the angel, falling angel, maybe that only makes him closer to what Sam needs, will give him. Cas gives a lot. His dream of Dean doesn't leave him deaf to the word Sam is begging against him.
Cas has let go of his shoulder. Sam feels sore and tired and guilty, but sated.
There's a bruise on Cas's throat. He's never bruised before. Sam almost reaches out to touch it. He stops himself, pulls away instead. "Sorry," he says. He doesn't look at Cas. He might gaze back at Sam with pity or worse, gratitude.
"It's alright." Cas answers, calm and unaffected. Sam makes himself stand, pick his shirt up off the floor. He can feel Cas staring at him, and wonders if today, Cas will finally want to talk about it. It'll spell the end of them. Sam can't look this head-on and continue to do what he's done. He tries to make it clear to Cas that he shouldn't say anything. Cas is blessedly silent, until finally, he also stands, picks up his pants and underwear where Sam had left them. "I should go."
Sam should offer to let him stay, at least. To rest. Cas looks so tired nowadays.
"Yeah," he says instead, and belatedly, "Thanks." It doesn't make up for anything he's done.
"Thank you, too." Cas says, stiff and awkward. He hesitates a moment longer. It's so quiet that Sam can hear the distant rumble of a lost car or two on the roads outside the motel. "Goodbye, Sam."
"Bye," Sam responds. Cas doesn't hear him. Cas is already gone. Sam is the only one still here in the aftermath.
If he manages to sleep tonight, his dreams will be empty. He doesn't know if that's a good thing or not anymore. He hates every part of him that feels lonely.