Well he certainly wasn't doing anything but vocally expression his delight at what Lucy was doing. If she could settle for that then maybe it wasn't onesided even though Natsu would rather do something for Lucy in return.

"Mmmmmn careful with that promise I might just want you to keep it no matter what" Natsu all but growled before leaning against that line of kisses Lucy drew over his neck. She was melting through his defense so easily and naturally.