I don't typically write this type of thing, so if I messed anything up, let me know.
If you asked him how he got to this point, Sam wouldn't really know what to tell you.
It had been a long, frustrating journey to understanding and acting on his feelings for Cas, and in turn, it had been a wild ride of communication problems and awkward attempts at physical anything before either of them had relaxed enough and trusted each other with the relationship…
But this was taking that trust and dialing it up to eleven.
Cas had been the one to bring it up, quietly and a bit awe-struck when he discovered it online for the first time. Sam had nearly died choking on his coffee, but when he was able to breathe again, they talked about it and… well…
"Not too tight, are they?"
Castiel asked breathily in Sam's ear, tugging gently at the restraints that held Sam's hands behind his back. With a slow exhale, Sam shook his head, his hair falling in its usual position over the blindfold that Cas had carefully wrapped around his head.
A kiss was pressed to his shoulder, and there was a quiet murmur of 'good' before he felt Cas adjust them so that Sam's back was against the headboard. He felt the warm pressure of Cas's weight against his thighs, then a caress against his cheek that pulled his head forward for a kiss. It was easy, simple, and it helped Sam get into the moment, that this was just them, that he could trust Cas implicitly.
When Cas pulled away with another lick of his lips, Sam heard the pop of a bottle of lubricant and the shlicking sound of fingers being prepped for entry. He felt Cas smile against his neck and whisper, "Don't tense up, it's not for you." It caused a wave of arousal to rock through him hotly, and he took a sharp breath to replace the moan he'd have made when Cas made a moan of his own, and Sam knew that he was fingering himself.
Somehow, it was really kind of hot to not be able to see, but to know exactly what he'd look like, the flush there would be across his skin- and it was sweet torture to not be able to express his pleasure at the prospect.
He tipped his head back and took some deep breaths as Castiel chuckled breathlessly and shifted on his lap, "Wishing… that it was you, love? Your fingers? Your, ahh, your dick?" Teeth made a mark where Sam's neck met his shoulder, and he bit his tongue to keep a sound at bay. His hips squirmed underneath Cas, his erection hot against his stomach, but he held back and kept the motion slight.
He was rewarded with soft licks and Castiel's nose nuzzling against his jaw, "Mm, very good, Sam." His name was a breathy moan on Cas's lips, and he ached to swallow it, but settled for licking his lips and counting his breaths.
Cas rocked against him while he prepared himself, a fantastic symphony of groans and shuddered breaths, and Sam knew he was ready when Cas let out a harsh gasp and arched against his chest, the man's other hand coming up to tangle in his hair.
"…I'm going to ride you, Sam," Castiel whispered to him, a quiet strength beneath the husk of his arousal, "Nice and slow. And if you're very good," He curled a finger through Sam's hair, "Maybe I'll let you come with me."
Sam shuddered and opened his mouth to a heavy exhale, which Cas kissed away, a smirk on his lips.
Suddenly Cas's hand enveloped Sam's cock to coat it in lubricant, and the taller man gave a jerk in surprise before being soothed down with kisses and gentle strokes. He bit his lip to keep from moaning at the gentle, teasing touches, and he felt the rumble of Cas's chuckle against his chest. "I rather enjoy seeing you like this," he stroked his free hand down the side of Sam's face, "Trying so hard to hold back when you're so painfully aroused…" He ran his thumb over the head of Sam's dick and relished in the little soundless moan Sam made before removing his hand from him completely.
There was only a slight amount of shifting before Cas was in position, and he curled one hand in Sam's hair and one on his thigh while he slid down Sam slowly, making the other man tip his head to his chest and breathe heavily through his mouth to not make much noise. Castiel shivered and groaned as he was filled, but didn't stop until he was fully seated in Sam's lap, and after giving himself a moment to adjust to the girth, he lifted himself back up without warning and then heavily back down, tearing a deep moan from his own throat and high gasp from Sam.
With that, Cas started up a rhythm, a slow but hard ride that had Sam trying uselessly to buck his hips into and caused a whispered, "Cas.." to escape him. Keeping the pace, Castiel tucked his head against Sam's neck and nipped at his skin, breath hot and tongue hotter, "Do you like that, Sam?"
Sam opened his mouth to reply, but then thought better of it and bit his tongue on a moan as Cas's hips rocked. With a hum, Castiel tugged Sam's head back by his hair and said, "Answer me, sweetheart," to which Sam could do little else but moan brokenly, "Yes, Yes, Cas, so good…"
Obviously pleased at the answer, Castiel kissed at Sam's jaw and increased his pace a bit, breathing little noises of pleasure against Sam's cheek and using his shoulders for leverage. It drove Sam wild with want. He wanted to put his hands on Cas's hips, to thrust into him and leave little bruises where his fingers had been, but the ties on his hands kept him from it, and instead there were Cas's hands against his shoulders leaving their own bruises, and he liked it.
He tried to keep quiet, and not moan much (though Cas, kind, gracious Cas let him slip by with some while they kissed, bless him), but he felt immediately when he started to get close, and let Cas know by tipping his head back and mouthing his name to him breathlessly. The message received, Castiel merely continued, digging his nails into Sam's shoulders and moaning a command, "Beg for it, Sam."
"Pl-ease," Sam panted, "please let me come in you, Cas," he tried, moving his hips as much as he could with the restraints, "Please, Cas, I'm so- you feel so good and I'm so close-"
"Hush, love," Cas murmured dazedly, and Sam bit his lip hard to stop the stream of praise and pleading. Still rocking hard against his lap, Cas took one hand off of Sam's shoulder to brace it elsewhere, achieving an angle that made him curse and writhe. Sam felt him tighten around him and groaned low in his throat, to which Cas kissed him roughly and practically growled against his lips, "Come, then. Now."
And what else could Sam do but come apart beneath him, tipping his head back and leaning against the headboard as his orgasm ripped through him. Cas was soon to follow, and Sam felt the wet heat against his stomach at the same time muscles clenched around him, which made him whimper.
When he'd ridden out his orgasm, Cas stopped, resting his head against Sam's and gingerly pulling off of him to rest. They both shivered with satisfaction, and after a moment of relishing the after-glow, Cas remembered Sam's ties and nuzzled against his cheek, "I'm going to untie you now."
Sam gave a weak nod, and when the blindfold was removed, he blinked his vision back, thankful for the dim light in the room. When his hands were released, he rolled his shoulders once before wrapping his arms around Castiel and smiling at him softly, "…Alright, you win."
Castiel grinned and gave him a sweet kiss, "I'm afraid I'm not the best at this whole thing, though."
"Good enough for me," Sam murmured against his lips, "We'll get better at it, both of us."
With a hum, Cas brushed his nose against Sam's, "That sounds like a promise."
"Maybe it is," Sam chuckled, "Ask my rope-burn in the morning."