The words nearly got a hollow laugh out of him, but it became strangled in his throat.
"My selfishness is what got you into this," he told her, not bothering to veil his bitterness. "You don't know--you don't know what it's like." He had been quite young when he was bitten, and the transformations had been frightening. To think of an infant going through the same thing, and knowing that it was his fault--his carelessness--he couldn't live with himself. He'd never dreamed of putting someone else through that much pain and suffering.
He broke a little, shoulders shuddering as he covered his face with his hand. Even if their child wouldn't inherit his affliction, it would be shunned for its father - something it couldn't even help.
"Love," he echoed, his voice a hoarse whisper. "James and Lily didn't know that Harry would suffer. Arthur and Molly's children are normal, with normal parents and father capable of supporting them. They are not me, Tonks. They are not my kind." He looked up sharply, meeting her eyes.
"It is terrifying," he agreed. "And the baby will suffer, I know it. If not because it's like me, then because I am its father." He turned away again, not wanting her to see the wretched look on his face. "I can't."