Their teeth are stronger than those of humans. They can crack wood with ease, can split bone, can chomp down fish and rats and stray pets during the hungry months. They can rip their enemies' throats, shimmer red with brutal endings. 
It's easy for Raph to take a bite out of Leo's shoulder--a bite as in a bite, with meat breaking away under his teeth and sliding into his mouth. The pain cuts like a knife and Leo moans, his cock already straining against the confines of his shell. 
"Tasty," Raph says around a mouthful of green scale and torn flesh. 
"Don't talk with your mouth full," Leo mumbles and Raph rolls his eyes before slipping in for another kiss. He wraps his teeth around Leo's mouth and tugs, ripping a chunk of his lip away. Pain blossoms out of Leo's mouth like a mouth, damp warmth staining his face. He gasps, spreading his legs wide, flushed and shaking with heat. 
"Better n' pizza," Raph muses, chewing happily. He lifts Leo up and turns him gently, sending his cheek sliding across the pillow. Leo cries out as Raph bites into his side, hard and deep enough to put a human at serious risk. But Leo isn't human, and Raph would have to devour him pretty deep to do something mutant healing couldn't fix. 
If he did go that deep, Leo doesn't know if he could stop him. Doesn't know if he would want to. Knows it doesn't matter, because he had already trusted Raph to not go too far long before their relationship took this turn. 
He drops down once Raph starts chewing at his hip, cock swelling and shaking frantically against the pillows. Raph laughs into the hole he's torn, teeth grinding against Leo's hip bone. It hurts so fucking bad, and Leo loves it so fucking much. He wails joyfully, hips thrusting into the air. 
"Bad boy." Raph hoists his bottom into the air. "You don't get to come except on my cock, gorgeous." The way he says it, in that hungry, gravelly voice of his, it sounds like the declaration of love Leo knows it is. 
He rips deep into Leo's thigh, again and again, crunching and tearing like he's starving, like he's never been full before. Ripping up the old flesh with his old scars, refashioning Leo to his design, and Leo will never be able to find words for how desperately he wants to be refashioned, because after so much time being broken by those he hates he would do anything to be shattered by someone he loves. 
A bite on one buttock, then the other, pieces of his ass ripped out in twin bright spurts. His flesh is softer here, and the scars will be deep enough for Raph to dig his fingers in later on, maybe even reopen them, again and again. Leo lets out a churr at the very thought, eyes rolling with pleasure. 
"Whore," Raph growls, spreading Leo's rim open with reddened fingers and taking a bite. Leo frantically grinds up into his brother's mouth, hunting his face with an animal's desperation. 
Raph yanks away with a hiss and sinks his teeth into Leo's tail, triggering an earsplitting squeak-scream. Leo feels his tail thrumming against the hot, wet caverns of his brothers' mouth, knows Raph could bite it all off if he wanted to, and Leo would let him. 
He doesn't--not tonight, anyway. He just drags his teeth along the whole quivering long, neatly shearing scales away and leaving Leo dripping red. He snaps off the very farthest tip and Leo almost blacks out with the pain, drops of precome sputtering from his cock. 
"Pain slut," Raph growls, spittle gently stinging against Leo's dripping wounds. "My darling little pain slut." He pushes Leo even higher, dragging rough fingers around Leo's erections. "I could carve ya up with my sai and eat ya, and you'd do love it, wouldn't you?" 
"Yeth," Leo slurs, words blurred by the state of his mouth and his fucked-up mind. It's okay, though--Raph understands, as he always has. 
He bites into Leo's cock and shakes, jerking Leo around hard enough he comes this close to tearing it clean off. Leo's hands scrabble automatically to get away even as he arches wildly up into the agony, into the sweet burning haze of being consumed, devoured, loved to the point of no return. 
"Raph," he sobs like a prayer, because his brother is the purest god he will ever find. A dark and jealous god, certainly, but that doesn't keep Leo from needing him with every fiber in his being. 
"It's okay," Raph whispers, slowly and torturously letting Leo's cock slip out over his teeth. "It's okay, dirty little darling. You're safe with me." And then he snaps a bite out of Leo's shaking, oversensitive dick--a small, small bite plucked off the side, but a bite, leaving a little dip in the flesh behind. 
Before pain has time to break through Leo's overwrought nerves, Raph slams his ass back down to the sheets, jerking Leo up into a sitting position as his ravaging underside scrapes the sheets. He squeaks and sobs like a crazy thing as red and white spurt from his cock, bones breaking to jelly at the most painful and most beautiful orgasm of his life. 
Raph doesn't give him time to breathe, let alone recover. He keeps dragging Leo back, back, back, free hand reaching up to drag across Leo's open wounds, dipping into each one while Leo jerks in his arms like a marionette. 
"Spread your legs," he hisses, and Leo shakes as he does what he's told, red dragging across the sheets like a murder scene. Raph's blood-slick fingers sink into his battered hole, ripping and shoving, opening him up with little more care than Raph would for a can of beans. 
When he shoves it in it's messy and brutal and painful, his dripping cock gouging Leo's hole before carving up his insides. Leo wriggles around the intrusion with half-strangled yelps and whimpers, blood dribbling down his chin to flow around his chest like a ruby necklace. 
"Mmm," Raph groans, sinking his teeth into Leo's undamaged shoulder, even deeper than before. More bites sprout as he rips pieces out of Leo's cheek and neck and shoulder, showing no signs of stopping, no end to his hunger. 
Raph chews and chews as he fucks Leo harder and harder, shoving and tearing at him from both ends. It's like he's been split down to the atom, swallowed, and spit up, reborn as something new, something wonderful.  
"I love you," Leo gurgles, gagging at the taste of his own blood and the hurting-happy tears spilling out of his eyes. "I love you, Raph." 
Raph grabs him by the cuts into his face and twists his head around at a viciously sharp angle, shoving their lips together. His teeth scratch along Leo's tongue, almost like he wants to rip it out and swallow Leo's words, keeping them all to himself. 
"I love you, too," he whispers into Leo's mouth, and it tastes like the monstrous grace Leo has been dreaming of for longer than he can remember.