Because no one likes to be helpless.
Ed was writhing into the mattress, his sole leg pushing down for leverage and his hand fisted in the colonel's shirt. His eyes were mad, breath gasping and short. Roy ran a gloved hand over his hip and between the junction of his thighs.
"Fuck, Mustang." The younger man bit his lip and buried his face into a pillow.
Roy heard the lust there but missed the tightness, the shiver in his voice. He kissed Ed's newly exposed neck, nipping and laving the taught muscle. Fullmetal sobbed and he removed the glove from his right hand. He lovingly traced a path around Ed's auto-mail port, before dipping towards his ass. He circled the fleshy nub he found with his index finger, savouring the gasps that were elicited.
The hand at his heart unclenched and disappeared from his vision. He looked up, puzzled, to see it covering his lover's face. He stopped his caresses and pulled himself higher. Ed was shaking and small.
"What?" Roy didn't understand, this had never happened before.
Ed had tear tracks running down his cheeks. He was biting his lip hard enough to draw blood.
"I can't fucking move like this," he hit the mattress in his frustration and to illustrate his point, "I can't push you away if I want to."
Roy blanched and he thought he might be physically ill.
"I'm sorry…"
Ed curled into him and cried in earnest. Roy cradled him loosely with one arm and felt that he deserved to die.