After having already kissed his partner and having Matt said all that he could (mostly with his hands and mouth), Karl wasn't worried so much about the leg pressed closer to him than he was about the look that she was giving him.
It was a familiar feeling, him being concerned about his partner's well-being and a welcome one, at that. Which meant that the small smile that he gave to her was 100% Karl and the belief he had in her was his as well.
He leaned forward and gave her a kiss on the corner of her lips.