His arm wasn't the only thing the Galra changed. They also gave him certain... needs. Shiro learns to make it work for him.

Magnus wants to reach down and run his fingers through Alec’s hair, but he can’t. His wrists are tied together with a length of silk rope that connects them to the headboard. They both know it’s only an illusion of control - Magnus is more than capable of getting out of the bonds with just a single thought and slightest push of magic - but it’s still a nice fantasy, having Magnus helpless like this. “Fuck,” Magnus whines, his hands straining against the rope with the overwhelming urge to touch, “Alexander, please!”