The best way to figure out how to have sex with an archangel, Adam has found, is to dive in headfirst.
This is literal, in most cases.
Adam never had the chance to go spelunking on Earth. He imagines if he had, he would have chickened out at some point. Too dark, too tight, too far below the surface for anyone to hear him yell for help. He's not really scared of any of those things anymore. His concept of claustrophobia has been blown out the water, and honestly, crawling on his belly through a tight channel of stone is more thrilling than terrifying. He wiggles his way deeper, nudging himself forward with his toes, shoulders scraping the walls on either side of him. When he began this descent, the passage was a little wider, enough for him to enter on his hands and knees, but it's closed in on all sides the further he goes.
If he'd thought about the ambience of a cave before this, he'd have answered that it would be made up of the echoes of his own breathing and slow dripping from deep within the cold stone. He's right about both of those. His fingers are damp from sliding in the trickles of water that escape through the cracks in the stone. His exhales puff foggily in front of his face before fading into the dark. Louder than both of those things is the hum that permeates the air around him. It caresses his cells, and it feels familiar, the pitch and frequency of it nearly matching the resonance that Adam usually contains. Adam squirms his way deeper inside. The hum beckons him in.
The passage grows tighter and tighter around him, squeezing until he moves by inches and then centimeters, but he never slows his expedition. When he thinks he will finally get stuck, that's when it begins to open up again.
He slides through the opening bit by bit. He ends up on his back, catching his breath, staring up into his glittering surroundings. What was cold gray stone expands into a huge cavern decorated with thousands of crystals. When he stands, he can see that it stretches out for miles around him. The hum is even louder here, rolling off of the crystals and the walls, engulfing him entirely.
In the back of his mind, a spare connection that Michael refused to sever reverberates with curiosity. The hum around Adam grows sharp in his ears, and he does his best to mentally quiet Michael again. Lucifer's not quite ready to share this intimacy with his brother. With his brother's vessel, on the other hand... Adam turns, admiring the crystals lining the walls and ceiling, and the hum evens out again into a low, pleased purr.
Most of them are various shades of pink and white. The tunnel leading inwards was so dark that Adam couldn't see a foot in front of his face, but light bounces between the crystals here. It's refracted so many times that Adam has no hope of figuring out its source. (Though the answer is obvious. The source is Lucifer himself. He has to be the shiniest bauble in the room at all times, even when he's the room.) Water pools in dips in the stone at Adam's feet, and when he pokes his foot in one curiously, it makes the tips of his toes go numb from the cold.
Adam treks further in. The crystals jut out from the walls in some incomprehensible pattern. Some are as small as Adam's palm while others stretch up the entire height of the cavern, melting seamlessly into another patch of crystals above him.
Adam reaches out a hand and lets it hover over a crystal that's hanging down from the ceiling. He's staring into the surface, but his own face isn't reflecting back at him. Instead, if Adam peers too deeply, he sees things that can only be described in simple terms, not because they lack complexity, but because they are so far removed from his own experience that the only similarities he can draw are from universal constants.
Before he even touches it, he can feel it vibrating, stirring the molecules around it. When Adam lets his hand drop to the smooth surface, it resounds up his arm. If he had to guess, he'd say that's a good reaction. He traces his hand down the crystal until it tapers off at chest height. Around him, the hum rises in pitch and steadies out again. Adam keeps walking, trailing his hands along crystals as he goes, hearing the walls echo with surprise. They are all chilly, silk-smooth, and impossibly bright.
Adam taps one. It rings out with a pure note. He taps it again to hear the same sound, and a different crystal provides him with another, lower note. He knows absolutely nothing about music, but he finds a rhythm that's pleasing to the ear, drumming his fingers against the rocks until the song he's making peals as loudly as the humming around him. It changes to match Adam's haphazard music-making, matching note after note, delight in a game that it, he, Lucifer, hasn't had the chance to play in millennia.
Adam's not sure he's finding any of this sexy, exactly, but it is interesting. He slaps a crystal larger than his whole body to hear it thrum. Leaving his hand there afterwards sends the vibration through his arm so strongly that it nearly knocks him to the floor.
Around him, the ambient frequency of the hum has sped and refuses to slow again. It's demanding in how it beats in Adam's ears. He drags his hand along a bright pink patch, sharp enough that they should cut his palm but he's earned the right to pet them without harm. With both hands, he finds another long crystal and rubs it. The beat within is pulsing, a powerful crescendo under his fingers, played to climax as Adam caresses the responsive stone. The cavern gleams, each of the crystals across every meter of the walls standing radiant. Adam shuts his eyes. He keeps rubbing with one hand, but the other beats out a tempo that speeds with the hum around him until-
It all crashes around him with the symphony of nuclear fusion, energy pouring out of every edifice. Adam could be lost forever in the harmony, but that lifeline Michael insisted on pulls tight.
When Adam can think straight again, Michael has dragged him back to a different perceptual plane, one where Lucifer wears a human body rather than approximating his true form in stonework. Michael curls around his soul, part protection and part jealousy when he feels how the molecules of his vessel are echoing with his brother's energy rather than his own. Lucifer lays still, breathing heavily, overwhelmed beyond measure. The sight of him shivering through the vestiges of an orgasm turns Adam on.
"So," he manages, "was it good for you?" Lucifer looks at him hazily. He doesn't even bother with a snarky reply, just lays back again and sighs.