It's a simple fact of life, one of those unalterable laws of the universe, that Loki does what he wants. He has motherfucking magical powers and the destiny to end the world and as much as Tony Stark wants to understand him, to pick him apart and analyze how he plays with the forces of the universe, he has to settle for going along with whatever Loki wants and pretending it was his idea in the first place.
Which is how, incidentally, he ended up with Loki beneath him, hands tied tight behind his back and legs splayed against Tony's shoulders. He has one of Tony's ties—one of the ugly ones Pepper bought him for some event or other—stuffed into his mouth and he's trying his best to talk around it but it's all muffled and broken because he has to breathe short through his nose. Tony's working his hands feather-light along the god's cock and Loki is beginning to squirm. If this was another night, he'd be hauling Tony down to him and biting at his lips and running those long, cold fingers down Tony's chest. But tonight all he can do is weakly lift his hips against Tony's fingers and shake his head back and forth like he's trying to spit out the gag. The lean musculature of his arms stands out as he strains against the length of smooth leather cording that binds his hands.
Tony bends and kisses the tip of Loki's cock, moving down to run his lips lightly over the shaft. He sucks on one of the god's balls and Loki's body vibrates as he moans. Tony moves back up to the head and swirls his tongue around it before taking as much as he can into his mouth. He places his hands on Loki's hips, holding him to the bed, as he starts to work the god's shaft in his mouth. He can feel the press of hips against his hands, the hot twitch of Loki's erection against his lips. He's staring at Loki while he works, watching the god's glass-green eyes splinter as want and desperateness builds behind them. He swallows around Loki's cock and Loki absolutely writhes.
He takes his hands off Loki's hips and slicks them with lube, trying to shift his weight as little as possible. He swallows around Loki at the same time he enters him, one finger, shallow. Loki pushes down forcefully, wriggling his shoulders against the mattress to get leverage, and Tony lets him. He adds another finger when Loki can take it; and another; and soon he has almost his whole hand going in time with his mouth and Loki is twitching and writhing and panting hot, wet, muffled moans into his gag. His glass-green eyes have long since closed and he's trying to push himself onto Tony as hard and as fast as he can. With his free hand, Tony slicks up his cock and trades it with his hand in a smooth, practiced motion, the hand that was preparing Loki coming around to wrap, slick and fast and steady, around his dick.
Loki screams an all-encompassing, body-wrenching scream that is a choked, broken "Stark" and Tony keeps going, fast, deep, keeping quiet and not touching Loki except for with his dick, his hands, and his shoulders where Loki's feet have constricted, drawn him forward. Fucking Loki always feels like this, like beating against walls of glass until they shatter, like riding the cosmic currents around a black hole, like infinity waits inside this feylike form, waiting for Tony to try to fill it.
He tries, every time. Sometimes with words, with tender touches and kisses and love, and sometimes with screams, with loud and rough sensation; he can tell there's a fissure there and he aches to stitch it shut any way that he can.
But Loki does what he wants. And Tony just has to go along with it and pretend it was his idea in the first place.
When the god comes, it's long and sharp and silent. His edges are dulled by the leather around his wrists, by the silk in his teeth. When Tony follows, it's hard and crashing and rough, like he's being stripped apart. It's always like this, fucking Loki. Like he's reaching for the abyss, giving all of himself that he can to fill it up, and seeing everything Tony shrink and fall away into the darkness at Loki's core.
He leaves Loki to lie afterwards, as per the god's request. He gets dressed in the other room and runs his fingers through his hair and contemplates infinity.