This is for a prompt on the Glee prompt meme I'm too lazy to look up now. I saw it some time ago and really wanted someone to write something for it, but no one did. So now I did it.
I'm not sure yet if this is going to remain a one-shot or if I will turn it into a multi-chapter story. It depends mostly on life and my trust in my ability to write this the way I want it to be. But if you have any preferences one way or the other, let me know :)
If you have questions, you can ask here or on tumblr. I love to hear from you!

Kurt turns the envelope in his hands a few times. It feels heavy, and something small and hard slides around the envelope as he moves it.

The letter is from Blaine, which is...unusual. They've texted some and talked a little since Thanksgiving, but things are still awkward. Kurt still feels a flash of pain whenever he thinks of Blaine, of what he has done - but he also feels something like hope about where they might go from here. He's glad he called Blaine that day. It was the best piece of advice Isabelle has ever given him.

And now, a letter. With something in it.

His curiosity wins, and he opens the envelope and fishes out whatever it is that is in there before it falls out and disappears in a crack in a floorboard or something.

It's a key. A very small, single key without a ring or any indication what it might unlock. He digs inside the envelope for a letter, for some kind of explanation.

The only thing left in the envelope is a small piece of paper with Blaine's handwriting on it.

This is the only key.

And he still doesn't know more than before.

He's torn between being annoyed and intrigued. He's not sure they're in a place again in their relationship where there's room for mysteries, but he can't deny he's curious, and so he gets out his phone and types a quick "Explain" and sends it to Blaine. Maybe that's all it's intended to be, a conversation starter; he just hopes he won't get back something cheesy like "It's the key to my heart." They're definitely not there yet, if they'll ever be. He's kind of made a resolution that if they ever get back together (and that doesn't mean they will), they'll have to take all the cheesiness and vows of love and eternity and replace them with honesty and communication.

What he gets back is a dick pic.

He assumes it's Blaine's dick, though there's not much of actual dick to be seen behind the bars of...something.

What is that? It looks like a cage, or something, made of plastic he assumes, though it's made up to look like metal. The space between the bars is wide enough to fit a finger in, maybe. It looks evil. It looks like the bearer might be able to pee and clean up with some practice, but do nothing more with his cock. Masturbate, maybe, with a lot of time and effort and if he doesn't mind the pain when his cock tries to get hard within that...thing.

Kurt's own cock twitches at the sight, though. He can't deny it looks appealing, the ring around the balls lifting them up for his viewing pleasure, the cock constrained.

He looks away. He won't get an erection from this, although he suddenly values his ability to get one more than just a moment before.

With trembling fingers, he calls Blaine.

…...

"It's a cock cage," Blaine says by way of hello. He hasn't been able to think of much else since he locked himself in two days ago so he could snail-mail the key to Kurt. He's constantly aware of the thing, though he thinks that today there might have been one or two moments he was actually able to focus on the lesson or whatever he was talking about with Sam or Tina.

He just hopes it works.

The worst thing is that he's weirdly turned on the whole time. Somehow, strangely, he likes the thought that no one can touch his cock, not even himself, except Kurt.

He can almost hear Kurt's eye roll.

"Yes, I know that, Blaine. The question is, why are you wearing it?"

Blaine takes a deep breath.

This is the moment that possibly decides whether he is going to get back together with Kurt. Which, of course, is why he is doing this.

"So I can't ever cheat on you again," he says.

Silence. He can hear Kurt's breathing, and his own heartbeat as it pounds in anticipation of what Kurt is going to say.

"Do you trust yourself so little?" Kurt finally asks.

"No," Blaine says, shaking his head for emphasis even though Kurt can't see it. "I know I'll never cheat on you again. But I know you don't, and I know you don't trust me."

Damn it. He has practiced this little speech, in front of the mirror no less, he shouldn't choke up now. But the thought of Kurt not trusting him...that he has betrayed Kurt's trust...it's too much.

"You...you locked yourself up so I can completely trust in the fact that you are not physically able to cheat on me again, even if you wanted to?" Kurt asks.

!Yes," Blaine says. More quiet, he adds, "You can also trust in the fact that I won't want to, but that's not so easy to prove."

"I'm not sure I'd call something like that easy," Kurt says, and Blaine grins.

"I kind of like it, actually," he offers.

"You do?" Kurt says. "I'd have thought it terribly uncomfortable."

"You get used to it," Blaine says. He hesitates, isn't sure if that's something he should share, if they're there yet. But he wants them to be, and if they're not, he knows Kurt won't hesitate to tell him so.

So he swallows drily and says, "I'm kind of turned on by it."

Silence. For so long that Blaine thinks it's definitely been a mistake to bring that up.

But then Kurt says, "Oh," and it sounds breathy and thin and not at all pissed off. "What...what do you like about it?"

"Um...I'm constantly aware of it. And," Blaine swallows. Is this going too far? Would Kurt be okay with..."So, this is...a lot. But, assuming you'd agree to let us be boyfriends again..."

"So that's what all of this is about," Kurt interrupts. To Blaine's relief, he sounds amused.

So, he bluntly admits, "Of course this is what it is about. Do you think I'd lock myself up for any goal less worthy?"

Into Kurt's laughter, he says, "So, assuming you'd agree to that, and you'd ever want to do... that again, I think it's...it's hot to think that you are the only one who can touch me there. I like that. It's like I belong to you."

…...

Oh God. It must be the most inappropriate reaction he could have had. He can't deny he likes the thought. Anyone who'd look in the area of his crotch could see he likes the thought. He opens the buttons of his pants, just for some relief, happy that Blaine can't see him. But he can't suppress a moan as his hand brushes his cock, and from the way Blaine's breath hitches, he has definitely heard that.

"Kurt, are you-" Blaine asks. "Are you touching yourself?"

Kurt snatches his hand away. "It's hot, okay?" he defends himself. "It's a hot thought. And since I can and you..."

You can't, he is about to say, but maybe that would be a little unfair.

Blaine takes a deep breath. "Do it."

"What?"

"M-make yourself come. Please. I want you to."

In a way, maybe, it makes sense. Blaine is – was - pronbably still is such a fan of masturbation it kind of makes sense that he'd want to listen to Kurt doing it. But they have never done that. A mildly suggestive text now and then, once they had started having sex. Never anything like what Blaine is asking for now. And they're not even together. Kurt hasn't thought about...although, let's be honest. He's had time to reflect on everything, and he knows that, though he isn't the one who cheated, he isn't completely innocent. He had neglected Blaine, had let New York dazzle him so everything had happened exactly the way Blaine had feared and Kurt had told him it wouldn't. So he has thought about getting back together with Blaine, once he could admit to himself that maybe he was to blame just the tiniest bit for how everything had turned out. Once he hadn't been so angry and hurt and completely devastated anymore. And, if we're completely honest, he has recently allowed himself to imagine things they could do if they were to get back together. Things that went beyond showing Blaine New York or singing duets.

Without thinking about it, he palms his cock.

"K-kurt?" Blaine asks, and Kurt realizes he hasn't said anything. He exhales softly, knows that Blaine must fear he has gone too far.

"I'm here," he says. He can barely get the words out. He is aroused to the verge of pain, but there is something else, too - the feeling that something monumental is about to happen, that he is standing at a threshold. If he does this...if he gets himself off while Blaine is listening on the phone, Blaine will expect that it means they're back together.

"I can't," he says. Blaine sighs, disappointed and insecure, and Kurt longs for him to be here so he can poke him in the side and tell him that's not what he means.

"I'm not saying I don't want to," he says. "I know that this is big, what you're doing, and I...I appreciate it. But I have to think about it, and I refuse to think with my dick."

Blaine laughs, a little shakily, and Kurt knows he's crying at the same time. It makes him want to change his mind, and his dick wants the same, but he resolutely closes his pants and his heart. He has to do this right. For his own sake, and for Blaine's, too, for nothing good will come of it if he finds out too late he hasn't forgiven him for cheating after all and then holds it against him at every little disagreement.

"I know what you're doing is...huge," he says. "And I don't want you to think I don't realize what you're willing to give up for me. I know you, after all."

Blaine chuckles with him this time, and Kurt is relieved.

"I'll make up my mind. Soon, I promise."

He forbids himself to jerk off that night, and the next day after school, he has made ten or so copies of the key that he's wearing - out of sheer paranoia, he tells himself, not for any reason more sentimental - on a chain around his neck. He can just imagine himself losing that key. He puts the keys in various places around the apartment, hopefully where Rachel won't find them, and swears that the next time he's in Lima, he'll hide one in Blaine's room for emergencies. Only then does he think that maybe he won't have to, because he'll send the key back to Blaine, with it rejecting the offer, the responsibility, and - well. Blaine.

He'd like to talk to Rachel, but she is too busy with school and Brody with the aaamazing abs (her words, not his) and all the wonderful new friends she's making and how impressed her teachers are...and she's doing, he realizes, the same to him that he did to Blaine. He very much wishes to have somebody else to talk to, someone who doesn't interrupt him all the time and instead just listens to him, and it makes him forgive Blaine a little more. Although he really, really didn't have to sleep with the guy. Well, you can't have him any more, random guy from Facebook, he thinks. He's mine.

Rachel tells him he's lucky to be single in New York, because apparently being single in New York is the best thing ever. The thing is just, Kurt doesn't feel single. He feels like he's the one holding the key Blaine gave him, together with the trust that they'd be able to forgive and make everything right again.

And he finds he's getting more and more ready to get started.

When he makes that decision, he's in bed, and it's really late. He can't call Blaine to tell him, and also doesn't want to tell him. Blaine's asleep, he won't suffer in ignorance, and Kurt can lie there and grin at the ceiling while he's contemplating the possibilities. Then he grins even wider as he has an idea how to break the news to Blaine. It's not very romantic, but he has a feeling Blaine might appreciate it.

He jerks off to images of Blaine begging him to take off the cage, to touch him, and it doesn't take long until he comes, stifling his moan with a pillow. Then he turns on the light, and takes the chain with the key from his neck and instead wraps it around his cock. He takes a picture of his dick, the key, and his cum-covered belly, and sends it to Blaine so he'll see it in the morning before school. Then he cleans up and goes to sleep.

And apparently, dick pics are now a thing they do. For the next morning, he gets one from Blaine, his locked-up dick an angry red and straining against the cage.

And a message: Call me tonight?