Draco liked to give him marks. Specifically scars.
Harry often thought he had enough of those, thank you very much, and if anyone else had done so, they would be kicked to the curb.
But there was something in that grey gaze, locked on his, as Draco dug the knife in a little deeper, sunk his teeth in a little harder, that showed Harry he couldn't live without this.
He'd never thought he'd be attracted to blood sports, pain and obedience, but he was. There was something about the dig of a blade into his flesh that calmed him, something about the slide of blood down his skin that soothed him. It was like a balm, and after a session, Harry always felt relaxed. There'd been a time when he wondered if he'd ever simply get to give up control, or would he have to be in charge forever?
Draco took that decision from him. He told Harry what to eat, where to sit, when to sleep. Harry appreciated that. Harry craved that.
What did Draco get in return? Control, when he usually felt he had none. And most of all, he got someone who trusted in him. Someone who needed him, more than he need them.
That was what their relationship was based on. Trust, and scars. When Harry'd come to Draco, his body had been pure and almost untouched. The scars he did have came from devastation, and pain. Emotional pain.
He'd set out to remedy that. Not by erasing those scars, but by adding more. Adding ones born of physical pain.
Draco enjoys running his hands down Harry's back. It's smooth no longer. It has bumps, ridges and valleys- all his doing. The public thought they were scars from the war.
Only they knew those were scars born of love.