in the experience I've had (a few FUDCons, a big commercial conference in SF, LFNW) drinking is very optional. If you want to go to a bar after the day is over, there will be some other people who do too. If you don't want to, there will be other people who will go to somewhere that isn't a bar. FUDCons have one officially-scheduled event which generally takes place in some sort of bar-ish environment, FUDPub, but it's a scheduled social event (not part of the actual 'business' bit of the conference), plenty of people go along and don't drink, and plenty of people don't go and still enjoy the rest of the conference.
So, I wouldn't shut out the idea of going to conferences entirely. Look at the schedule for the ones that interest you and see if they seem to be heavy on social events that happen in bars. Maybe ask a few people who've been before. But certainly, in my experience, F/OSS conferences tend to be pretty non-drinker-friendly.