Someone was going to hear that and come knocking, right? Charlie hadn't eaten a cupcake and so he had enough sense to realize that she was quite loud. Charlie also had Penny thrusting back against him and therefore had limited capacity to worry about it for too long.
He wrapped his hands around her hips and pulled her back against his body. He found the idea of someone hearing them excited him a little, so long as he imagined it being anyone but a family member standing outside the door.