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"Dean, your scrotum appears to be tightening" says Cas, gently ghosting his fingers over the spine of Dean's illiac bone.
"Your cremasteric reflex is quite amusing," he continues, nuzzling at his suprapubic region, then dragging his nails over the same spot, watching in fascination as his fingers elicit the same automatic response.
"Oh my fuckinggod, Cas, what is wrong with you, just... do the thing."
"What thing, Dean?" he asks, leaning in and licking at the glans experimentally, feeling his ischiocaverne muscle contract with want, filling him up with blood.
"C-Cas! The thing… where you, ah... take my dick into your mouth for chrissake, stop teasing me," he gasps, tightening his grip in Cas' hair.
Cas just blinks at him, feigning ignorance and starts gently ghosting over his perineal area with his fingers. He circles his scrotum and trails up the inside of his thigh until he can feel the pulse of his femoral artery, strong and quick and hot under his fingers.
"Human penis is such an interesting organ, Dean." he says, licking again at the base of it, contemplating the blood rushing into both cavernose bodies, filling them up and making them rock hard and waiting, waiting for any kind of friction. He gently pulls at the delicate skin until he exposes the sensitive glans, thumbing it lightly over the top. Just a bundle of nerves concentrated on this small percentage of skin, but capable of such intense pleasure.
He finally leans in and starts sucking in earnest and it's an interesting experience, a rhythmic motion that sets Dean's nerves on fire, he can feel it thrum under his skin, a slow crescendo of physical force translating into neuronal impulses shooting straight at Dean's brain at 35 meters per second until he's seizing and shooting something else entirely down Cas' throat. Symphatetic division takes over the parasympathicus and Dean finally relaxes with a sigh.
"Jesus Cas, that was fucking amazing, man..." he breathes, trailing his hands through Cas' hair, combing it behind his ear absentmindedly.
Cas looks thoughtful for a second, then drags himself up Dean's body, leans in and gives him a quick peck on the lips. "Dean, your semen's quality is exceptional. A little high on the fructose, perhaps. You're a very virile man."
"Shut up, Cas..."