I don't usually drink (could get through perhaps 1/4 of a glass of wine in an entire evening, and only if it's a rosé), but I attend the afterparties anyway. I've not been pressured at any of them to change the fact that I don't drink. The last one, I got a shocked reaction when I had a sip from a friend's wine glass.
Geeks seem to be able to carry on semi-intelligent conversations (or at least technological debates) even after a few beers (unlike the people I go to school with), so they can still be pleasant company. I've never seen a drinking game at a con party, only drunk games, by which I mean drunk people playing games that are unrelated to drinking--trivia games, board games, Nintendo, etc. The majority know their limit and how to control themselves, even inebriated.
It is, of course, also possible to not attend the parties while still attending the presentations. The booze doesn't come out for about 2 hours afterward.