The kiss was probably ruined a little at first by the fact that Clint absolutely had to laugh over the threat. It was very Jamie - all that tough love because he cared. Clint liked it. Of course he did. He kissed back though, putting his all into it -- even as his fingers wrapped around the clasps of Jamie's belt just so he could have some better balance.
And because, well, he wasn't about to lose sight of his original task here.