Oh my gosh! I soooo hear you! For the longest time, I was in the same boat, since there are really so many jerks out there who will try to make you feel inferior. Well, y'know what? Being fat and out of shape is FIXABLE. Learning to not be a jerk is much more difficult!
Go out there, have a great time, and don't let the losers get you down. They're only responding from their own insecurities and looking for someone to pick on. Sod them!