21st birthday drama

I turn 21 in a few weeks and my parents are taking me out to get drunk as a rite of passage. My girlfriend of almost nine months is incredibly against this. She was always against alcohol but we had a couple of good times drinking with friends before. I thought she was finally okay with it but one night she threw up and began to swear against it again. Worse than ever. She keeps saying that she doesn’t want me to drink ever. It has caused a couple of arguments already and I feel terrible. It would be one thing if I was an alcoholic or even drank every weekend but that has never been the case in our relationship. Am I an asshole for arguing against her wishes?