Em, how about a simple "Unless Naruto somehow turns himself into a real, functional woman and lets me knock him up, you're the only one. ...shut up." I think it'll make Sakura squiggle more because of the unintended fluff and phrasing. And it sounds mushy enough and his feelings are thrown off enough that he'll be upset since he'll realised what he said only after he said it.

Also I forgot to add:
"I don't know how to stop," he said, very quietly.
I see this as more 'brokenly' then 'quietly'. >.<