Sets some new rules for the arrangement

You are caught red handed desperately beating your throbbing cock while thinking of what Carrie was doing with your sister. Just imagining your girlfriend's toned body and big tits servicing Sam was taking you right to the edge near instantly. The particularly honeyed words your girlfriend laid on your sister's massive endowment still fresh in your mind as you furiously abused yourself. You'd have busted in a few more seconds if you hadn't been interrupted by the storyteller herself.

Regardless of how explicit it was, Carrie seemed pretty keen on being the one to give you your next orgasm, and the frown on her face confirms that in her eyes you've gone far off script.

"Absolutely not," Carrie says, "I'm going out of my way to do something super weird for you, and you can't even keep it in your pants long enough to let me have fun with it?" She admonishes. Her grimace and tone seeming oddly sincere despite her recent playfulness.

"Well from the sounds of things you seemed pretty into this deal yourself. I think we're both getting more than our fair share out of this arrangement." You retort, half pleading and half poking fun.

Carrie rolls her eyes at that, "Oh really? Don't even try to play that card. You came more thinking about how much I enjoyed your sister's huge cock then you ever have just having sex with me. And not five minutes after I got done wanking you to the tale, I catch you beating yourself senseless without me even present. I left you hard for a reason you know?"

She exhales slowly and then continues, "Listen, I agreed to bed your sister both because I thought it would be nice for her and as a favor to you. But at the end of the day it was a bit of fun for us to share. I'm not going to go fuck your sister and then come back to nothing, I'm your girlfriend, not your whore. You wanking yourself flaccid on your lonesome is making me lose out on half the fun."

You immediately recall how obviously interested Carrie had become in your sister's cock after just the first time she saw it, and how she accelerated this whole situation near entirely on her own. But you think better of vocalizing this in your current position. Staying mum seems the best choice as an oddly playful grin crosses Carrie's face and she spins on a heel away from you.

She's been fresh out of the shower this whole time, her toned dripping body shines with precipitation and blushes from heat as she saunters over to her purse. You are still harder than you have ever been in your entire life, and your girlfriend's glistening peach dancing as she bends over does little to change that.

"Don't worry, I wouldn't want to call this off now." She chirps as she giddily skips back over to you, some hidden prize clutched tightly in her fists. "It would be an awful shame to disappoint Sam at this point, and I know how much this means to you." She explains, voice dripping with implication.
"Luckily, from the moment you suggested something like this, I prepared for just such an occasion. This is just a little something to make sure this stays fun for both of us."

Carrie's grin widens as her palms upturn to reveal her secret treasures. In one hand is small key, and the other is what appears to be a small metal ring tightly affixed to an even smaller rounded metal disc with a hole in it.

You look at the device, confused for a moment. Honestly having no idea what this has to do with sex at all.

Carrie, picking up on your bafflement, explains, "It's a chastity cage, silly. I thought someone whose so into being humiliated would know one when they saw it."

You simultaneously blush and throb at the jab but nevertheless respond, "I can kind of see that now, but where even is the cage part? As it stands I don't know if I could even fit in that thing if I wanted to."

"Hm? The little cap part is the cage of course. I knew I couldn't trust you to keep your hands to yourself, so I made sure to get the smallest tightest one I could find to keep your dick as 'sheltered' as possible." She taunts mischievously. "And you better make it fit, because either you wear this for as long as I ask, or I pump the breaks on our deal."

You're suddenly in a panic. You had never thought about just how much this arrangement had meant to you until now, and the answer was apparently enough to actually risk considering this. It started as just a pick-me-up for your awkward sister, but your girlfriend's 'updates' on the matter quickly transformed this into something else entirely, both because of Carrie's thoughts on Sam, and just how hot you found it considering those details.

Sleeping deep inside your sister was a monstrously hung sex god just waiting to be let out, and the thought of her fucking someone like Carrie was the hottest thing you have ever imagined. Your hind brain was screaming at you to protect that beautiful image at all costs.
It had to happen, but this little **** device looked like it would be a serious pain.

Reasoning with her, you plead, "Listen Carrie, I get it. I'll try to keep my hands off myself so we can continue having our little pow-ow sessions. They are really fun for me too, I just don't think a cage that small is safe. I even heard using them can shrink your cock. Permanently even! If there's an ounce of truth to that myth, that tiny thing would be the one to do it."

"Aww, are you afraid that your sister's Big, Thick, Powerful, Perfect, Cock is going to be so much that I just never want to go back to your shriveled little dicklet ever again?" She tuts huskily, all the while ever so gently tracing her index finger up the underside of your turgid shaft. "I promise you it already more than puts this little thing to shame, the difference is already so vast that I probably wouldn't even notice a change."

Each word beats your psyche like a drum, and your diamond hard erection can take no more. You burst right there on the spot, a legendary load by your own standards but apparently only giggleworthy to Carrie now, as she angles your modest member's wasted decree towards your body with just that single finger.

"Hahaha, apparently someone isn't that bothered by the idea after all!" She remarks with a devilish grin.
Your cock quickly goes soft for the first time in the past two hours, and she doesn't waste a beat. Quickly pulling your balls through the ring and smushing your finally flaccid little pecker into the thumbnail sized cage.

Much to your chagrin, your soft dick fits inside rather easily, and with a damning snap your manhood is tightly packed away.

Noticing your grimace, Carrie clicks her tongue and says, "Don't worry baby, I promise to let you out right after the next time I play with your sister's beautiful monster cock. And who knows, if you can find a good hiding spot, this time I might even let you watch."

Your imprisoned little nub visibly quivers at the prospect of witnessing your sister's legendary lance in action, and Carrie catches its dew with her finger.
She sucks it off with a cheeky wink then proceeds to get dressed and ready for her next encounter. And you are left in the room feeling frustrated as your caged member pointlessly tests the confines of its new home.

This is going to be dangerous...