You seem to be a bit ignorant. You know the phrase if you can't beat them join them? I'm not saying to start drinking the thing that ruined your life because there is another phrase I enjoy, it's "If you don't like it get the fuck over it."
It doesn't sound like your girlfriend was flaunting it, it sounds like she drank a few beers...
There is no issue in someone else drinking a few beers, the issue is what you are making it, the issue is you.
If someone else wants to make a choice and drink you should be fine with that because it's not your choice it's theirs and has nothing to do with you. If you flip out at your girlfriend for drinking a few beers I hope she laughs at you.