Actually some of my sincere christian friends never drink alcohol. But I don't mind. It's their own choice and belief. So I think it's OK among your friends but when it comes to business or among colleagues who don't know you enough the situation could be li~~ttle bit different. Drinking is kinda common culture to mingle together in Korea and this tend to happen more often among men. But don't worry :) If you excuse yourself explaining your situation mostly they'll understand you. It's not a big deal I think :D

I think shunned is a very strong word. I am allergic to alcohol ao I can't drink but I don't feel pressure to drink. Once they know it is for health reasons, I think that people are usually quite accomodating. On the other hand, drinking (especially with co-workers) together is a popular and cherished past-time of Koreans - so they may feel a little bit awkward in this situation. If you make them feel at ease, then there should be no problems.