He does respond well to rewards, and she's always been good at providing them. Of course he's going to keep pushing the boundaries, when she's being so amenable -- encouraging, even. He doesn't even bother to glance over his shoulder to see if anyone's watching them. The party's noisy, and people're drunk. Nobody'll take notice of the fact that he's currently deepening the kiss as much as he can and sliding that exploring hand over her ass. Apparently, he likes what he finds, from the little hum of appreciation he makes into her mouth.