lots of drinking is a large part of most tech related events. opensource events are far from being special.
drinking is not forced on anyone, and as a non-drinker I have never been even teased at opensource events for not drinking (although I have had it happen at some other more 'professional' events)
after hours the local bars are a common place for people to hang out and talk, and during this time most people drink. some people get falling-down drunk, but the vast majority do not (I will note for the record, that the Europeans tend to be heavier drinkers than the Americans in my experience, but that's a trend rather than a divide)