avocado

I go to bars on a regular basis to see music, hang with friends, get a bite to eat late night or just to go somewhere to sit and chill and talk with people. I don't really see how it can be possibly construed as 'impolite' if I order a diet coke instead of a beer. If it's an issue for you, I suspect it's not really the 'politeness' aspect but something else - temptation, maybe, or self-consciousness. If being in bars affects you negatively like that, I'd recommend avoiding them until you can work out exactly what the problem is.