Most children are born without heart nor bone marrow. As they leave the womb, they must have tubes inserted to them, leading out from their parent, so that one heart can beat harder for two, one skeleton fill two bodies. Even if the other body is a baby, the mother still takes a toll. Her chest aches. But it must be she alone who bears this.
Most, as they grow, and as they learn to look around, and as they learn to speak, grow hearts of their own, and their bones hollow and fill with blood. After a time, the mother's heart is able to beat like it once did, and some time after, maybe the arteries and veins connecting them are removed, only returning for the occasional transfusion.
Unless the child is lazy, and ungrateful, and even as their own heart grows, they do not spur it to beat, but continue taking from their parent - even as they continue to grow.
After a time, after they have grown enough, the parent will say "that's enough", and cut the tubes, and mend the skins. The child, still, will refuse to spur their heart to beat, and find someone else. Some foolish soul who will look into their faux-desperate eyes, and see their skin turning green, and hear their gasps, and think it to be real, and choose with ever-foolish love to commit the taboo and beat for them, too.
...
And then the person you love will take only as much blood as she needs.
You can still hear her strained breaths. You can see how, sometimes, the light doesn't quite reach to the edges of her eyes.
You can feel how she tries to be as little of a burden on your own chest as possible. She doesn't want to cause you that pain too. The one that her parent so bemoaned. The aching pain you can feel, faintly, even now. The way one of a weak heart might get dizzy. And she doesn't quite believe that she deserves to bleed.
She doesn't run. She only moves just as much as she has to. She doesn't dare lift a finger unless it's towards her plate - and even then, she cries and apologizes half the time.
Maybe it'd ache so if she did. But wouldn't it be nice to see her skin a healthy color for once?