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It's not so much that this is where Bruce means to be going from the outset, as that he got here, and didn't want to stop.
The arc reactor looks cold, blue and frigid in Tony's chest, but it's warm and alive, and Bruce wonders how Tony stands it because as soon as he throws his knee across, as soon as he has Tony pinned, his knees wide on the floor over Tony's arms almost at his sides and his ass resting just against Tony's chest, as soon as his balls touch the transparent surface of the device, every tiny hair on them vibrates like they've been pulled just taut and strummed, and Bruce shudders and stills.
It's like goose bumps, only no chill and no fear, just the whisper of a quiet hum between his legs, nothing like a vibrator, and he's already hard because obviously if there is near-naked pinning going on, it's going somewhere involving fucking, right? Well, or something; they usually don't get much past unzipped. But Tony looks up at him and smirks, and goddamn if he doesn't lick his lips and ask, "Good?"
And yeah, it's good. Bruce strokes himself once, watching Tony watch him, and he wants to lean forward, to bring his dick over Tony's lips and feel the slick suction he knows Tony would apply, all along the underside and around the tip and hey, Tony's always good for a throat-roughing blowjob but no, no, he stays where he is, and he nods. "Yeah. Good."
Tony nods. "I feel it, like, sort of focusing."
"You... what?" Bruce is stroking slowly, enjoying the resistance as every upward tug pulls his balls (and their hairs) free of the surface and then lets them all back down to electrify again. "It's not sentient."
"Yeah, no, I know. It's usually more generalized," Tony says. "You're providing an external focus--not that I fucking care because hi, look at you, but I'm just saying, it feels like it's paying attention to you."
"..Okay. So, I should...?"
"Do exactly what you're doing, baby." Tony lifts his head off the floor despite that his arms and shoulders are pinned, straining for a taste of Bruce's dick, and Bruce lets him have one, just a kiss at the tip, and then he's jerking again, and Tony's grinning. "Gonna come on my face?"
Bruce always likes to do that, likes the way Tony relaxes under the drip of semen, grinning broadly, head back and chin up, and he likes it even better when Tony looks forward to it, so he slows his hand again. "Not yet," he says. He knows there's no way he's holding off; every slower stroke means there's a longer half-second for his balls to buzz against the reactor, means the sensation low in his belly builds all the faster as though something is twisting there in the most appealing way possible, and he can see in Tony's eyes that he knows it. "Well," he allows. "If you put your head down and let me do the work."
Tony puts his head down, but he licks his lips again as he watches, and his body is rocking slightly, like he's rutting up against the air behind Bruce and Bruce remembers, he's still wearing underwear, ridiculous lacy thong underwear that only Tony Stark would wear for no real reason. He must be getting some friction out of the movement, must be getting ready to come, too.
"I'm being good," Tony says. "Excellent, even. What're you doing to make it worth my while?" And Bruce doesn't have a good answer for that, but he pushes his knees apart a little wider, sits down a little more firmly, and feels the almost-buzz up into his ass, through his balls up into his shaft, against the soft flesh that covers the tendons of his inner thighs and he moans a little and jerks faster.
"What'm I doing?" he asks. It's inevitable now, any second, so he just says, "I'm coming on your face, Tony. I'm holding you down and squirting on you like I've wanted to for months. What're you doing for me?" His timing is good, anyway; on the last word his dick jumps in his hand and he's spurting thick and white in Tony's beard, on his nose, on his lips.
Tony shudders under him and grunts, "Coming in my pants, babe."
Bruce grins and keeps stroking, wringing out every drop, then falls forward on his hands, scoots back, rubs his ass on Tony's wet thong, and presses his tongue into Tony's mouth. He tastes come and spit, and Tony's hands are in his hair, and the sensation of the reactor against his chest is as cool and ordinary as it seems like it should be.
Tony wipes his face with his fingers, sucks them clean, and mutters, "I think I gotta make some more adjustments to this thing. Because that one--okay, it wasn't for this, but it went even better than I expected."
Bruce chuckles, kisses him again, and agrees.