You don't need any reason! Tell them to screw off! Tell them your in-training to win fights, and make something of yourself...Unlike them. I go to bars, clubs, house parties, and I have never took a sip. People ask me why I don't, but I tell them I don't need it to have fun. I train like hell so usually most people understand by looking at me without asking.
I got to a stage where I don't put anything in my body unless it fuels performance, so I don't eat empty calories. Alcohol is an empty calorie and a toxin.