They fight like cats and dogs at school, and it doesn't stop when Santana fucks Tina. The goth-styled girl's kinks don't just stop at whips and spikes; when she brought out the knife, Santana was actually stricken dumb for a few moments. She experimentally scratched Tina's face; Tina retaliated by etching a long, thin red line over Santana's back.
After that, it escalated. Santana can't get off without scars. Tina is only too happy to oblige. Sometimes she uses her nails; lately, it's been a sharpened silver knife, sprung up from its enamel casing by an almost silent flick of a button. Tina experimentally flicks and sheathes the knife, watching Santana's eyes follow the gleam in anticipation.
Tina licks Santana, her tongue tracing the hot, salty blood. That tinge of metal leaves her wanting more, and Santana's pained, half-horrified, half-amazed gasp encourages her to try a little more cutting. Soon, the cheerleader has several dark lines, oozing blood that runs in gleaming stripes down her back. Tina's tongue licks over the wounds, her saliva causing a stinging that's almost too great to bear – but that causes Santana to arch herself against the other girl in pleasure.
Pain as pleasure – neither girl would have imagined it. It's only now, when they lie in bed together, blood sticking their bodies together, stinging cuts made worse by each other's salty sweat, that they realize how much they have in common.
It doesn't do to go into backgrounds. You don't have to have been abused in order to enjoy pain and blood. However, though Tina wasn't, Santana was, and that's why she insists on it. Later, when she cries in bed and Tina holds her close, letting her lie on her small breasts, she doesn't ask questions and Santana doesn't offer any sob stories. Instead, she'll let the tears drip onto Tina's nipples, then she'll bite them, sucking hard, the exquisite pain and shot of warmth to Tina's groin making her wet and uncomfortable until Santana deems to fuck her again.
Tina keeps her nails long because Santana likes it to hurt. When Tina inserts her fingers into Santana's pussy, she pushes, hard, against the resistance and hears the other girl gasp horribly. She knows Santana likes it to hurt, and she's made her bleed more than once. It's such a rough, painful relationship, but it serves them both.
After the hurt, Santana will hold Tina against her, listening to the other girl's ragged, painful breaths; listening to the heartbeats slow, feeling limbs stop shaking. And they'll fall asleep with blood on the sheets, with rusty stains on skin and stuck between body parts. The entire room will smell like blood.
The next day, they'll go to school, nursing secret pain, and ignore each other until the next time.
Until it hurts so much that there needs to be release. That is, obviously, why knives and kinks were invented.