Marty had started drinking at home and was already more than a little buzzed. His mom cared even less than his dad did about him stealing a bottle or two from the liquor cabinet and he took full advantage of that.
When Sheila greeted him and Robert sarcastically, he rolled his eyes at Robert and turned to Sheila. "I believe that's my line," he said, leaning over to give her a quick kiss on the cheek.