You might think you become wittier with every sip of scotch you take, and your girlfriend might agree, but we guarantee you her parents won't. If there was ever a time to show restraint with the booze, it's when her parents are the ones supplying it.
Chugging a glass of Cabernet to calm your nerves, while totally understandable is, as Sublime would say, the wrong way. Their first impression of you shouldn't be an image of you drooling between telling inappropriate jokes. There's a time and place for dick jokes and slurring, and it's not with her family.